Systems and methods of triggering interference mitigation without resource partitioning

ABSTRACT

Systems and methods for triggering interference mitigation at a wireless device in a cellular communications network are disclosed. In one embodiment, a node associated with a cellular communications network makes a determination to trigger interference mitigation at a wireless device based on a signal load in at least one interfering cell from which transmissions result in interference at the wireless device during reception from a desired cell, a relation between reference signals used in the desired cell and the at least one interfering cell, and a timing relation between at least one of a group consisting of: signals transmitted by the desired cell and the at least one interfering cell and signals received at the wireless device from the desired cell and the at least one interfering cell. In response to making the determination, the node triggers interference mitigation at the wireless device.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/178,696, filed Feb. 12, 2014, now U.S. Pat. No. 9,420,476, whichclaims the benefit of provisional patent application Ser. No.61/766,996, filed Feb. 20, 2013, the disclosures of which are herebyincorporated herein by reference in their entireties.

FIELD OF THE DISCLOSURE

The present disclosure relates to interference mitigation at a wirelessdevice in a cellular communications network and, more specifically,triggering interference mitigation at a wireless device in a cellularcommunications network.

BACKGROUND

In cellular communications networks such as 3^(rd) GenerationPartnership Project (3GPP) Long Term Evolution (LTE) networks, there aretwo types of deployments, namely, a homogenous network and aheterogeneous network. A homogeneous network utilizes a single layer, ortier, of radio network nodes. In one particular example, all radionetwork nodes in a homogeneous network are High Power Nodes (HPNs) suchas wide area base stations serving macro cells. As another example, allradio network nodes in a homogeneous network are Low Power Nodes (LPNs),e.g., local area base stations serving pico cells. When there aresimilar load levels in the different cells of a homogeneous network, awireless device, which is sometimes referred to as a User Equipmentdevice (UE) or terminal, typically receives equally strong signals froma serving or measured cell and from a closest neighboring cell(s),especially when the UE is located in the in the cell border region.Therefore, in a homogeneous network, resource partitioning betweenserving and neighboring cells for the purpose of inter-cell interferencemitigation is not as critical as in heterogeneous networks.

A heterogeneous network includes two or more layers of radio networknodes. In particular, each layer of the heterogeneous network is servedby one type, or class, of Base Stations (BSs). In other words, aheterogeneous network includes a set of HPNs (e.g., a set of high poweror macro BSs) and a set of LPNs (e.g., a set of low power or mediumrange, local area, or home BSs) in the same geographical region. A BSpower class is defined in terms of maximum output power and other radiorequirements (e.g., frequency error, etc.) which depend upon the maximumoutput power. The maximum output power, Pmax, of the BS is the meanpower level per carrier measured at the antenna connector in a specifiedreference condition. The rated output power, PRAT, of the BSs fordifferent BS power classes is expressed in Table 1 below.

TABLE 1 Base Station rated output power in LTE (FDD and TDD) BS classPRAT Wide Area BS — (note) Medium Range BS <+38 dBm Local Area BS ≤+24dBm Home BS ≤+20 dBm (for one transmit antenna port) ≤+17 dBm (for twotransmit antenna ports) ≤+14 dBm (for four transmit antenna ports) <+11dBm (for eight transmit antenna ports) NOTE: There is no upper limit forthe rated output power of the Wide Area Base Station.As stated above some of the requirements may also differ between BSclasses. For example, as shown in Table 2 below, the frequency error isworse for LPNs. The frequency error is the measure of the differencebetween the actual BS transmitted frequency and the assigned frequency.

TABLE 2 Frequency error minimum requirement in LTE (FDD and TDD) BSclass Accuracy Wide Area BS ±0.05 ppm Medium Range BS  ±0.1 ppm LocalArea BS  ±0.1 ppm Home BS ±0.25 ppmA wide area BS serves a macro cell, a medium range BS serves a microcell, a local area BS serves a pico cell, and a home BS serves a femtocell. Typically, a wide area BS is regarded as a HPN, whereas all theremaining classes of BSs can be regarded as LPNs.

In a two layer macro-pico heterogeneous network, the macro cell and picocell layers typically include wide area BSs, which are also known asmacro BSs, and local area BSs, which are also known as pico BSs,respectively. The high data rate wireless devices located close to thepico BSs (i.e., in the pico layer) can be offloaded from the macro layerto the pico layer. A more complex heterogeneous deployment may includethree layers, namely, a macro layer, a micro layer that is served bymedium range BSs, and a pico layer. An even more complex heterogeneousdeployment may include three layers, namely, a macro layer, a picolayer, and a home or femto layer.

Heterogeneous networks, and in particular the co-channel scenarioutilized by heterogeneous networks, bring more challenges in terms ofmanaging interference. For example, inter-cell interference experiencedby the UE in the downlink and by the BS in the uplink needs to bemitigated. To address this issue, Inter-Cell Interference Coordination(ICIC), Enhanced ICIC (eICIC) and Further eICIC (FeICIC) techniques havebeen developed in 3GPP. The eICIC and FeICIC techniques are time domainschemes in that they enable interference mitigation by virtue ofresource partitioning in the time domain between the aggressor, orinterfering, cell and the victim cell. This in turns partly or fullymitigates the interference towards the victim cell, or more specificallyat the receiver of a victim wireless device in the victim cell.

According to the time domain eICIC or FeICIC schemes, subframeutilization across different cells is coordinated in time throughbackhaul signaling, which for LTE is backhaul signaling over X2connections between BSs. Subframe utilization is expressed in terms of atime domain pattern of low interference subframes or “a low interferencetransmit pattern.” More specifically, these low interference transmitpatterns are referred to as Almost Blank Subframe (ABS) patterns. TheABSs are configured in an aggressor cell (e.g., a macro cell) and areused to protect resources in subframes in the victim cell (e.g., a picocell) receiving strong inter-cell interference. The serving BS signalsone or more measurement patterns to inform the UE about the resources orsubframes that the UE should use for performing measurements on a targetvictim cell (e.g., a serving pico cell and/or neighboring pico cells).These measurement patterns are more specifically referred to as timedomain measurement resource restriction for the Primary Cell (PCell) andtime domain measurement resource restriction for the neighbor cells.Each measurement pattern includes of bit map of subframes (e.g.,10000000) where 1 indicates a subframe that is available formeasurements and 0 indicates a subframe that is not available formeasurements. Typically, there are 1-2 restricted subframes per radioframe since traffic density in an LPN is much lower compared to that ina HPN. Examples of measurements are Reference Signal Received Power(RSRP), Reference Signal Received Quality (RSRQ), Channel StateInformation (CSI) (e.g., Channel Quality Indication (CQI), RankIndicator (RI), Precoding Matrix Indicator (PMI), etc.). While there canbe measurement restriction patterns, there are not such patterns forrestricting scheduling of a UE. As such, typically, a UE is alsoscheduled in restricted subframes which overlap with low interferencesubframes (e.g., ABS) in aggressor cells. Therefore, UEs experiencebetter signal quality in these subframes.

In cellular networks, a wireless device is normally configured to reporta CQI to a serving BS and thereby indicating a Signal-to-Interferenceplus Noise Ratio (SINR) observed by the wireless device in the downlinkfrom the serving BS. Based on this CQI report, the serving BS selects asuitable Modulation and Coding Scheme (MCS) to be used when transmittingdata to the wireless device in the downlink. The wireless devicetypically derives the CQI by first estimating the downlink channel ofthe serving BS and then estimating the interference and noise as theresidual obtained by removing an estimated desired signal from thereceived signal. Interference estimation in LTE for CQI estimation isperformed over a set of predefined, or configured, Resource Elements(REs). In LTE Release 8 (Rel-8) to Release 10 (Rel-10), interferencemeasurements are expected to be done on REs carrying a Cell-SpecificReference Signal (CRS), whereas in LTE Release 11 (Rel-11) dedicatedInterference Measurement Resources (IMRs) were introduced in conjunctionwith LTE transmission mode 10.

In 3GPP LTE networks, downlink transmissions are based on OrthogonalFrequency Division Multiplexing (OFDM) in which physical resources canbe seen as a time-frequency grid of REs, where physical channels andsignals are mapped to specific REs. One type of downlink physical signalrefers to the CRS, which is used for demodulation of data as well as formobility measurements and CQI estimation. The CRS is regularlytransmitted by all cells, and the structure and locations of the CRS inthe time-frequency grid are known after cell acquisition. The density ofCRS symbols depends on the number of configured antenna ports. In LTE, acell can be configured with 1, 2, or 4 antenna ports. The locations ofthe CRS symbols can be shifted in the frequency domain where theparticular shift is given by the physical layer cell identity. Indeployments with more than one antenna port, three frequency shifts canbe considered. In LTE, a downlink subframe can be configured as“Multicast-Broadcast Single-Frequency Network (MBSFN)” which means thatCRS is not present in the data region of the subframe. As CRSs arecommon to all wireless devices in a cell, CRSs are not precoded and arealways transmitted with full power.

For LTE transmission modes 1 to 9, interference measurements as part ofderiving CQI are expected to be done on REs carrying CRS of the servingcell. These interference measurements are then used to predict theinterference on REs carrying data. The accuracy with which theinterference measurements on the REs carrying CRS reflects theinterference on the data depends on both the CRS locations and thetraffic load of the interfering neighboring cells, i.e. the aggressorcells. In time synchronized LTE networks, CRS transmissions in aggressorcells may either interfere with serving cell REs carrying CRS or REscarrying data, depending on CRS frequency shifts among the cells. Thus,this implies CRS-to-CRS collisions across cells when a non-shiftedconfiguration is used in all cells of a synchronized network. On theother hand, CRS-to-CRS collisions across cells can be partially avoidedif shifted CRS configurations are used among cells. However, CRS-to-CRScollisions can in general not be completely avoided with only threefrequency shifts, and sometimes non-shifted configurations can bepreferred from a user throughput perspective in low-to-medium loadedtraffic scenarios.

In a time synchronized network scenario with two dominating aggressorcells, the inter-cell interference on the CRS REs in the cases ofnon-shifted and shifted CRS configurations can be expressed as:

${I_{CRS} = {\underset{\underset{{LOAD}\_{INDEPENDENT}}{︸}}{I_{CRS}^{{NC}\; 1} + I_{CRS}^{{NC}\; 2}}\mspace{14mu}\left( {{non}\text{-}{shifted}} \right)}},{and}$${I_{CRS} = {\underset{\underset{{LOAD}\_{DEPENDENT}}{︸}}{I_{DATA}^{{NC}\; 1} + I_{DATA}^{{NC}\; 2}}\mspace{14mu}({shifted})}},$where I_(CRS) ^(NCx) and I_(DATA) ^(NCx), for x=1,2, represent the(averaged) interference caused by Neighbor Cell (NC) CRS and datatransmissions, respectively. In the non-shifted scenario, the measuredinterference refers to only the CRS transmissions of the neighbor cells.Since CRSs are transmitted with full power, a wireless device willmeasure high interference independently of the traffic load in theneighbor cells in the non-shifted scenario. Thus, such interferencemeasurements can only be representative in scenarios where the aggressorcells are highly loaded. In contrast, in the shifted scenario, themeasured interference refers to data transmissions of the neighborcells, and the interference level observed by a wireless device wouldthen depend on the traffic load in the aggressor cells. As the purposeof the interference measurements is to predict the interference level ondata, it can be noticed that the non-shifted case will typicallyover-estimate the interference level whereas the shifted case willunderestimate the interference level as the CRS based interferencemeasurements would not capture the impact of the CRS transmissions ofthe aggressor cells as illustrated by following expressions:I _(DATA) =I _(DATA) ^(NC1) +I _(DATA) ^(NC2) (non-shifted), andI _(DATA) =I _(CRS) ^(NC1) +I _(CRS) ^(NC2) +I _(DATA) ^(NC1) +I _(DATA)^(NC2) (shifted).

From the expression I_(DATA) (shifted) it is evident that there will beinterference on the data even when there is no scheduled downlinktraffic in the aggressor cells, i.e. I_(DATA) ^(NCx)=0. However, the CRSrepresents only a fraction of the REs within a resource block (roughly10%) so the relative impact of the CRS interference on the totalinterference depends on the traffic load in the aggressor cells. As thetraffic load in the aggressor cells increases, the impact of the CRS onthe total interference decreases. More specifically, as the traffic loadincreases, the CRS represents a smaller fraction of the totalinterference and, as such, the impact of the CRS on the totalinterference decreases. As in the non-shifted case, the interferencemeasurements will reflect the interference level on data most accuratelywhen the aggressor cells are highly loaded.

In LTE Rel-11 under the FeICIC Work Item, support for InterferenceCancellation (IC) by the wireless device on CRS REs (IC-CRS) wasintroduced. The wireless device has the capability of removing a numberof interfering (or aggressor) cells on those REs. The amount ofaggressor cells that can be removed is up to two, but in principle itcan be any positive number upper bounded by the number of discoveredinterferers. By applying IC-CRS, channel estimation performance can befurther improved due to less noisy signal samples in the non-shifted CRScase. In addition, CRS inter-cell interference on data REs can befurther reduced in case of shifted CRS configuration. Furthermore, inorder to simplify the IC-CRS implementation on the wireless device side,network assisted Radio Resource Control (RRC) signaling was introducedin LTE Rel-11. With this signaling, the serving cell informs thewireless device of the physical layer cell identities and correspondingnumber of antenna ports of up to eight potential aggressor cells. Whenthe wireless device has acquired this information, the wireless deviceknows the locations of the CRS in potential aggressor cells withoutautonomously detecting these locations.

In an LTE Rel-11 co-channel heterogeneous network deployment, a largeCell Range Expansion (CRE) of up to 9 Decibels (dB) is supported. When awireless device is in the CRE region of an LPN (e.g., a pico, micro, orfemto/home BS), the received signal at the wireless device can beinterfered by up to two strong macro aggressor cells. Therefore, in thisscenario, the received SINR (aka Synchronization Channel (SCH) Ês/lot orCRS Ês/lot) at the wireless device served by the LPN when located in theCRE region of the serving cell can be very low, e.g. down to −11 dB. TheSCH herein includes one or more of Primary Synchronization Signal (PSS)and Secondary Synchronization Signal (SSS).

In order to correctly detect received signals, the wireless device inthe CRE region has to cancel interference on certain physical signals(e.g., CRS, PSS/SSS) and certain physical channels (e.g., PhysicalBroadcast Channel (PBCH)). To facilitate interference cancellation ormitigation of these physical signals and/or physical channels at thewireless device, a radio network node can assist the wireless device byproviding a list of assistance data as specified in 3GPP TechnicalSpecification (TS) 36.331 for Release 11:

 NeighCellsCRS-Info-r11 ::= CHOICE {    release  NULL,    setup CRS-AssistanceInfoList-r11  } CRS-AssistanceInfoList-r11 ::= SEQUENCE(SIZE (1. . maxCellReport)) OF CRS- AssistanceInfo  CRS-AssistanceInfo::= SEQUENCE {    physCellID-r11   PhysCellID    antennaPortsCount-r11  ENUMERATED {an1, an2, an4, spare1},    mbsfn-SubframeConfigList-r11  MBSFN-SubframeConfigList  }  -- AN1STOP RadioResourceConfigDedicatedfield descriptions neighCellsCRSInfo This field contains assistanceinformation, concerning the primary frequency, used by the UE tomitigate interference from CRS while performing RRM/RLM/CSI measurementor data demodulation. The UE forwards the received CRS assistanceinformation to lower layers. When the received CRS assistanceinformation is for a cell with CRS colliding with that of the CRS of thecell to measure, the UE may use the CRS assistance information tomitigate CRS interference (as specified in [FFS]) on the subframesindicated by measSubframePatternPCell, measSubframePatternConfigNeighand csi-MeasSubframeSet1. Furthermore, the UE may use CRS assistanceinformation to mitigate CRS interference from the cells in the IE forthe demodulation purpose as specified in [FFS].According to the above Information Element (IE), the CRS assistance datacontains list of aggressor cells, their antenna port information, andtheir MBSFN configuration.

It has also been specified in 3GPP TS 36.133 V11.2.0 that the wirelessdevice shall meet the measurement requirements when the wireless deviceis provided with CRS assistance information, which is valid over themeasurement period. Therefore, the reception of the CRS assistance dataat the wireless device is used by the wireless device to perform the ICon, e.g., CRS, PSS/SSS, etc. However, in a heterogeneous networkdeployment, the wireless device typically applies IC on restrictedsubframes indicated in measurement patterns, which are signaled to thewireless device by the serving radio node via RRC protocol as describedabove.

RSRP and RSRQ are two existing radio measurements performed by thewireless device. RSRP and RSRQ measurements are used for at least RadioResource Management (RRM) purposes such as, e.g., mobility, whichincludes mobility in the RRC connected state as well as mobility in theRRC idle state. RSRP and RSRQ measurements are also used for otherpurposes such as, e.g., enhanced cell Identity (ID) positioning,Minimization of Drive Test (MDT), etc.

RSRP and RSRQ measurements can be absolute or relative. An absolutemeasurement is performed on signals from one cell, e.g. a serving cellor a neighboring cell. A relative measurement is the relative differencebetween the measurement performed on one cell and on another cell, e.g.between a serving cell measurement and a neighboring cell measurement.

CSI measurements performed by the wireless device on the serving cellare used by the network for scheduling, link adaptation, etc. Examplesof CSI measurements are CQI, PMI, RI, etc.

The radio measurements performed by the wireless device are used by thewireless device for one or more radio operational tasks. One example ofsuch a task is reporting the measurements to the network, which in turnmay use them for various tasks. For example, when in the RRC connectedstate, the wireless device reports radio measurements to the serving BSof the wireless device. In response to the reported measurements, theserving BS takes certain decisions, e.g. it may send a mobility commandto the wireless device for the purpose of a cell change. Examples ofcell change are a handover, an RRC connection re-establishment, an RRCconnection release with redirection, a PCell change in CarrierAggregation (CA), Primary Component Carrier (PCC) change in PCC, etc. Inthe RRC idle or low activity state, one example of a cell change is acell reselection. In another example, the wireless device may itself usethe radio measurements for performing tasks, e.g. cell selection, cellreselection, etc.

In order to support different functions such as mobility (e.g., cellselection, handover, etc.), positioning a wireless device, linkadaption, scheduling, load balancing, admission control, interferencemanagement, interference mitigation, etc., a radio network node (e.g., aBS) also performs radio measurements on signals transmitted and/orreceived by the radio network node. Examples of such measurements areSignal-to-Noise Ratio (SNR), SINR, Received Interference Power (RIP),Block Error Ratio (BLER), propagation delay between a wireless deviceand itself, transmit carrier power, transmit power of specific signals(e.g., Transmit (Tx) power of reference signals), positioningmeasurements, etc.

In a multi-carrier or CA system, a wireless device is served by multipleComponent Carriers (CCs), which are also sometimes referred to as cellsor serving cells. The term CA is also called (e.g., interchangeablycalled) “multi-carrier system,” “multi-cell operation,” “multi-carrieroperation,” or “multi-carrier” transmission and/or reception. CA is usedfor transmission of signaling and data in the uplink and downlinkdirections. One of the CCs is a PCC, which may also be referred tosimply as a primary carrier or even an anchor carrier. The remainingCC(s) are referred to as a Secondary Component Carrier(s) (SCC(s)) orsimply a secondary carrier(s) or even a supplementary carrier(s).Generally, the PCC carries essential wireless device specific signaling.The PCC, which is also known as the PCell, exists in both uplink anddownlink directions in CA. In case there is single uplink CC, the PCellis obviously on that CC. The network may assign different PCCs todifferent wireless devices operating in the same sector or cell.

Therefore, in CA, the wireless device has more than one serving cell inthe downlink and/or in the uplink: one serving PCell and one or moreserving Secondary Cells (SCells) operating on the PCC and SCC(s),respectively. The PCell is interchangeably referred to as a PrimaryServing Cell (PSC). Similarly, the SCell(s) is(are) interchangeablyreferred to as a Secondary Serving Cell(s) (SSC(s)). Regardless of theterminology, the PCell and SCell(s) enable the wireless device toreceive and/or transmit data. More specifically, the PCell and SCell(s)exist in the downlink and uplink for the reception and transmission ofdata by the wireless device. The remaining non-serving cells on the PCCand SCC are called neighbor cells.

The CCs belonging to the CA scheme may belong to the same frequency band(for intra-band CA), different frequency bands (for inter-band CA), orany combination thereof (e.g., two CCs in band A and one CC in band B).Inter-band CA including carriers distributed over two bands is alsoreferred to as Dual-Band-Dual-Carrier-High Speed Downlink Packet Access(DB-DC-HSDPA) in HSPA or inter-band CA in LTE. Furthermore, the CCs inintra-band CA may be adjacent or non-adjacent in frequency domain. Thenon-adjacent case is referred to as intra-band non-adjacent CA). Ahybrid CA including intra-band adjacent, intra-band non-adjacent, andinter-band is also possible. Using CA between carriers of differenttechnologies is also referred to as “multi-Radio Access Technology (RAT)CA,” “multi-RAT-multi-carrier system,” or simply “inter-RAT CA.” Forexample, the carriers from Wideband Code Division Multiple Access(WCDMA) and LTE may be aggregated. Another example is the aggregation ofLTE and Code Division Multiple Access (CDMA) 2000 carriers. Yet anotherexample is the aggregation of LTE Frequency Division Duplexing (FDD) andLTE Time Division Duplexing (TDD) carriers. For the sake of clarity, CAwithin the same RAT can be regarded as “intra-RAT” or simply “singleRAT” CA.

Multi-carrier operation may also be used in conjunction withmulti-antenna transmission. For example, signals on each CC may betransmitted by the BS to the wireless device over two or more antennas.Further, the CCs used for CA may or may not be co-located in the samesite or BS or radio network node (e.g., relay, mobile relay, etc.). Forinstance, the CCs may originate (i.e., be transmitted/received) atdifferent locations (e.g., from non-co-located BSs, or from a BS and aRemote Radio Head (RRH) or Remote Radio Unit (RRU)). Examples ofcombined CA and multi-point communication include Distributed AntennaSystem (DAS), RRH, RRU, Coordinated Multi-Point (CoMP), multi-pointtransmission/reception, etc.

Several positioning methods for determining the location of a targetdevice, which can be a wireless device, a mobile relay, a PersonalDigital Assistant (PDA), or the like may be used. These methods include:

-   -   Satellite based methods: Satellite based methods use Assisted        Global Navigation Satellite System (A-GNSS) (e.g., Assisted        Global Positioning System (A-GPS)) measurements for determining        position of a target device.    -   Observed Time Difference of Arrival (OTDOA): OTDOA methods use        Reference Signal Time Difference (RSTD) measurements for the        target device to determine the position of the device in LTE.    -   Uplink Time Difference of Arrival (UTDOA): UTDOA uses        measurements done at a Location Management Unit (LMU) to        determine the position of a target device.    -   Enhanced cell ID: Enhanced cell ID based methods use one or more        of UE Receive (Rx)-Tx time difference, BS Rx-Tx time difference,        LTE RSRP/RSRQ, High Speed Packet Access (HSPA) Common Pilot        Channel (CPICH) measurements, Angle of Arrival (AoA), etc. to        determine UE position. Fingerprinting is considered to be one        type of enhanced cell ID method.    -   Hybrid methods: Hybrid methods use measurements from more than        one method for determining UE position.        In LTE, the positioning node, which is also known as an Evolved        Serving Mobile Location Centre (E-SMLC) or location server,        configures the wireless device, BS, or LMU to perform one or        more positioning measurements. The positioning measurements are        used by the wireless device or the positioning node to determine        the location of the wireless device. The positioning node        communicates with wireless device and the BS in LTE using LTE        Positioning Protocol (LPP) and LPP A (LPPa) protocols,        respectively.

SUMMARY

Systems and methods for triggering interference mitigation at a wirelessdevice in a cellular communications network are disclosed. In oneembodiment, a node associated with a cellular communications networkmakes a determination to trigger interference mitigation at a wirelessdevice based on a signal load in at least one interfering cell fromwhich transmissions result in interference at the wireless device duringreception from a desired cell, a relation between reference signals usedin the desired cell and the at least one interfering cell, and a timingrelation between at least one of a group consisting of: signalstransmitted by the desired cell and the at least one interfering celland signals received at the wireless device from the desired cell andthe at least one interfering cell. In response to making thedetermination, the node triggers interference mitigation at the wirelessdevice. By triggering interference mitigation in response to theaforementioned conditions, interference mitigation is substantiallyimproved.

In one embodiment, the desired cell is a serving cell of the wirelessdevice. In another embodiment, the desired cell is a measured cell ofthe wireless device.

In one embodiment, the node is the wireless device such that making thedetermination to trigger interference mitigation at the wireless deviceand triggering the interference mitigation are each performed by thewireless device. Still further, in one embodiment, the wireless devicereceives an indication from a network node of the cellularcommunications network that the wireless device is to performinterference mitigation and, in response, makes the determination totrigger interference mitigation at the wireless device.

In one embodiment, the node is a network node of the cellularcommunications network such that making the determination to triggerinterference mitigation at the wireless device and triggering theinterference mitigation are each performed by the network node. Further,in one embodiment, the network node is a radio access node. In oneparticular embodiment, the radio access node is a base station of aserving cell of the wireless device. In one embodiment, the network nodetriggers interference mitigation at the wireless device by providing animplicit indication to the wireless device to perform interferencemitigation. In another embodiment, the network node triggersinterference mitigation at the wireless device by providing an explicitindication to the wireless device to perform interference mitigation.

In one embodiment, the node makes the determination to triggerinterference mitigation at the wireless device when predefined criteriaare satisfied, where the predefined criteria are based on the signalload in the at least one interfering cell, the relation between thereference signals used in the desired cell and the at least oneinterfering cell, and the timing relation. In one embodiment, thepredefined criteria include: a first criterion that the relation betweenthe reference signals used in the desired cell and the at least oneinterfering cell be non-colliding, wherein the non-colliding referencesignals do not overlap in time and frequency, and a second criterionthat the signal load in the at least one interfering cell be less than apredetermined threshold. In one embodiment, the interference mitigationis Cell-Specific Reference Signal (CRS) interference mitigation, and therelation between the reference signals is a relation between CRSs usedin the desired cell and the at least one interfering cell.

In one embodiment, the relation between the reference signals used inthe desired cell and the at least one interfering cell includes acolliding or non-colliding relation between the reference signals usedin the desired cell and the at least one interfering cell, wherein thecolliding reference signals overlap (fully or partly) in time and/orfrequency. Further, in one embodiment, the reference signals are CRSsused in the desired cell and the at least one interfering cell. In oneembodiment, the node makes the determination to trigger interferencemitigation when predefined criteria are satisfied, wherein thepredefined criteria are based on the signal load in the at least oneinterfering cell, the relation between the reference signals used in thedesired cell and the at least one interfering cell, and the timingrelation, and the predefined criteria include a first criterion based onthe colliding or non-colliding relation between the reference signalsused in the desired cell and the at least one interfering cell.

In one embodiment, the node triggers the interference mitigation suchthat the interference mitigation is performed by the wireless deviceduring channel estimation, and the node makes the determination totrigger interference mitigation when predefined criteria are satisfied,wherein the predefined criteria are based on the signal load in the atleast one interfering cell, the relation between the reference signalsused in the desired cell and the at least one interfering cell, and thetiming relation, and the predefined criteria include a first criterionthat the relation between the reference signals used in the desired celland the at least one interfering cell be non-colliding.

In one embodiment, the node makes the determination to triggerinterference mitigation when predefined criteria are satisfied, whereinthe predefined criteria are based on the signal load in the at least oneinterfering cell, the relation between the reference signals used in thedesired cell and the at least one interfering cell, and the timingrelation, and the predefined criteria include a first criterion that thetiming relation be such that the wireless device can perform theinterference mitigation using a single receiver.

In one embodiment, the node makes the determination to triggerinterference mitigation when predefined criteria are satisfied, whereinthe predefined criteria are based on the signal load in the at least oneinterfering cell, the relation between the reference signals used in thedesired cell and the at least one interfering cell, and the timingrelation, and the predefined criteria include a first criterion that thetiming relation be less than a predetermined threshold.

In one embodiment, the node makes the determination to triggerinterference mitigation at the wireless devices further based on one ormore supplemental criteria.

In one embodiment, the one or more supplemental criteria include asignal operation type of the wireless device. In one embodiment, thesignal operation type is one of a group consisting of: channelestimation, interference estimation, demodulation assessment, andChannel State Information (CSI) assessment.

In one embodiment, the signal operation type of the wireless device isinterference estimation followed by channel estimation and the at leastone interfering cell includes two or more interfering cells. Further, inthis embodiment, the node makes a determination to trigger interferencemitigation at the wireless device for at least one of the two or moreinterfering cells that is inactive prior to interference estimation andto trigger interference mitigation at the wireless device for at leastone other of the two or more interfering cells that is active afterinterference estimation and prior to channel estimation. In response,the node triggers interference mitigation at the wireless device for theat least one of the two or more interfering cells that is inactive priorto interference estimation and interference mitigation at the wirelessdevice for the at least one other of the two or more interfering cellsthat is active after interference estimation and prior to channelestimation in response to making the determination.

In one embodiment, the one or more supplemental criteria include abattery life of the wireless device. In another embodiment, the one ormore supplemental criteria include power consumption at the wirelessdevice. In another embodiment, the one or more supplemental criteriainclude a network deployment scenario of the cellular communicationsnetwork. In another embodiment, the one or more supplemental criteriainclude a location of the wireless device with respect to the at leastone interfering cell. In another embodiment, the one or moresupplemental criteria include a frequency error between the desired celland the at least one interfering cell.

In one embodiment, a node associated with a cellular communicationsnetwork includes a processor configured to make a determination totrigger interference mitigation at a wireless device based on a signalload in at least one interfering cell from which transmissions result ininterference at the wireless device during reception from a desiredcell, a relation between reference signals used in the desired cell andthe at least one interfering cell, and a timing relation between atleast one of a group consisting of: signals transmitted by the desiredcell and the at least one interfering cell and signals received at thewireless device from the desired cell and the at least one interferingcell. The processor is further configured to trigger interferencemitigation at the wireless device in response to making thedetermination to trigger interference mitigation at the wireless device.

In one embodiment, the node is the wireless device. In anotherembodiment, the node is a network node of the cellular communicationsnetwork. In one embodiment, the network node is a radio access node.Further, in one embodiment, the radio access node is a base station of aserving cell of the wireless device.

Those skilled in the art will appreciate the scope of the presentdisclosure and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 illustrates one example of a cellular communications networkoperating according to one embodiment of the present disclosure;

FIGS. 2A and 2B illustrate one example of colliding reference signalsand non-colliding reference signals, respectively;

FIG. 3 is a flow chart that illustrates the operation of a nodeassociated with the cellular communications network to triggerinterference mitigation at a wireless device according to one embodimentof the present disclosure;

FIGS. 4A and 4B illustrate two examples of performing interference andchannel estimation at a wireless device;

FIG. 5 illustrates a process for triggering interference mitigation at awireless device when performing interference estimation and channelestimation according to one embodiment of the present disclosure;

FIGS. 6A and 6B illustrate two examples of the process of FIG. 5according to one embodiment of the present disclosure;

FIG. 7 is a functional block diagram that illustrates the operation of awireless device according to one embodiment of the present disclosure;

FIGS. 8A through 8C illustrate three example embodiments of the presentdisclosure;

FIG. 9 illustrates an embodiment that is similar to that of FIG. 8A butwhere obtaining of at least some of the information used for decidingwhether to trigger interference mitigation at the wireless device isexplicitly shown as being obtained from the wireless device and the basestation of one of the interfering cells according to one embodiment ofthe present disclosure;

FIG. 10 is a block diagram that illustrates functional components of thebase station of the desired cell, the base station of one of theinterfering cells, and the wireless device according to one embodimentof the present disclosure;

FIG. 11 is a block diagram of an example embodiment of a base station;and

FIG. 12 is a block diagram of an example embodiment of a wirelessdevice.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the embodiments andillustrate the best mode of practicing the embodiments. Upon reading thefollowing description in light of the accompanying drawing figures,those skilled in the art will understand the concepts of the disclosureand will recognize applications of these concepts not particularlyaddressed herein. It should be understood that these concepts andapplications fall within the scope of the disclosure and theaccompanying claims.

Before describing embodiments of the present disclosure, the followingdefinitions are beneficial.

Radio network node: As used herein, the non-limiting term “radio networknode” is used to refer to any type of network node serving a wirelesscommunication device (e.g., an instance of a User Equipment device (UE))and/or connected to other network node(s) or network element(s).Examples of radio network nodes include a Base Station (BS), aMulti-Standard Radio (MSR) radio node such as MSR BS, a node B, anenhanced Node B (eNB), a network controller, a radio network controller,a BS controller, a relay, a donor node a controlling relay, a BaseTransceiver Station (BTS), an Access Point (AP), etc.

Network node: As used herein, the non-limiting term “network node” isalso used to refer to any type of radio network node or any network nodewhich communicates with at least a radio network node. Such nodes maynot themselves necessarily be capable of wireless communication.Examples of network nodes are any radio network node stated above, acore network node (e.g., a Mobile Switching Centre (MSC), a MobilityManagement Entity (MME), etc.), an Operations and Management (O&M) node,an Operation Support System (OSS), a Self Organizing Network (SON), apositioning node (e.g., an Evolved Serving Mobile Location Centre(E-SMLC)), Minimization of Drive Test (MDT), etc.

UE or wireless device: The terms UE and wireless device (or wirelesscommunication device) are used interchangeably herein. As used herein,the non-limiting term wireless device is used to refer to any type ofwireless device capable of communicating with a radio network node in acellular or mobile communication system. Examples of a wireless deviceinclude a target device, a device-to-device UE, a machine type UE or UEcapable of machine to machine communication, a Personal DigitalAssistant (PDA), an iPad®, a tablet computer, a mobile terminal, a smartphone, a Laptop Embedded Equipment (LEE), a Laptop Mounted Equipment(LME), a Universal Serial Bus (USB) dongles, etc. In particularimplementations, the wireless devices described herein may not support afull range of conventional communication capabilities and may provideonly a subset of the capabilities supported by conventional wirelessdevices. For instance, the wireless devices described herein may only becapable of communication in a particular direction (e.g.,uplink/transmission only, downlink/reception only) or may only becapable of communicating specific information or types of information.Examples of such wireless devices may include wireless meters orsensors, wireless-capable equipment, and Radio Frequency Identification(RFID) tags.

Interference mitigation: The terms “interference mitigation receiver,”“interference cancellation receiver,” “interference suppressionreceiver,” “interference rejection receiver,” “interference awarereceiver,” “interference avoidance receiver,” etc. are interchangeablyused but they all belong to a category of an advanced receiver or anenhanced receiver. Interference cancellation or suppression by suchadvanced receiver structures can lead to the elimination of theinterference, in which case the interference is completely cancelled,whereas in other cases the impact of interference on the useful signalis reduced. Interference mitigation refers to the receiver's ability tomitigate (i.e., cancel, suppress, or otherwise mitigate) theinterference caused by at least certain signals received at the wirelessreceiver from at least one interfering cell. An interfering cell is alsoreferred to herein as an aggressor cell.

Interfering cell: A cell, which is a neighbor to the serving cell of thewireless device or to any cell measured by the wireless device, and fromwhere the wireless device receives at least certain type of interferingsignals. An interfering cell is also interchangeably referred to hereinas an aggressor cell or a dominant neighboring cell.

The present disclosure relates to interference mitigation in a cellularcommunications network (which may also be referred to as a wirelesscellular network or simply a cellular network). Embodiments aredisclosed for network assisted interference mitigation by exploitinginterference mitigation capabilities in the wireless device, or UE,side. Among other things, interference mitigation can be used to assistsignal demodulation at the receiver of the wireless device and toimprove channel quality estimation.

The embodiments described herein focus on interference mitigation in thedownlink direction. However, the embodiments described herein can beextended to additionally or alternatively provide interferencemitigation in the uplink direction as well. In addition, some of theembodiments discussed herein focus on 3^(rd) Generation PartnershipProject (3GPP) Long Term Evolution (LTE) and, in particular, toCell-Specific Reference Signal (CRS) and CRS Interference Cancellation(IC-CRS) from LTE Release 11 (Rel-11). However, the embodimentsdescribed herein can be generalized to any suitable system (e.g., anysuitable cellular communications network and/or any suitable type ofreference signals and associated interference mitigation). Certainaspects of the disclosed embodiments are especially advantageous tosystems that utilize overlapping pilot signals for interferenceestimation/mitigation and support interference cancellation/mitigationmechanisms on those pilot signals. Furthermore, the embodimentsdisclosed herein can be generalized to cover any other future wirelessstandard where interference estimation takes place.

Accurate interference estimation is a very complex and important processtaking part in both signal demodulation and Channel QualityIndication/Index (CQI) estimation for link adaptation. Accurateinterference estimation means that the receiver can deduct theinterference correctly from the total received signal and hence improvedemodulation performance. In addition, the receiver can provide a goodestimate of the radio environment and thus select an appropriateModulation and Coding Scheme (MCS) that can maximize the linkperformance.

Typically, interference estimation is done on Resource Elements (REs)where a serving cell does not transmit data. In LTE Releases 8 up to 10,interference estimation for CQI estimation is based on REs carrying theCRSs. If the CRS positions in the frequency domain are non-shifted withrespect to CRS positions of interfering neighbor cell(s), then a ratherhigh bias is introduced in the interference estimation, which leads tounacceptable overestimated interference levels. This is especially thecase when a traffic level in the interfering cell(s) is low (i.e., inlow loaded networks). In LTE Rel-11, the possibility of IC on the CRSREs (IC-CRS) in the UE terminal side was introduced. IC-CRS can indeedlower the aforementioned bias. Nevertheless, there are cases whereIC-CRS can increase the gap between actual and estimated interference.For example, in the case of a fully loaded network, applying IC-CRS andremoving some interferers on the CRS REs will lead to a situation whereinterference is underestimated. Hence, there is a need for systems andmethods that assist interference estimation by appropriately utilizingthe IC-CRS UE capability.

Further, in general, inter-cell interference mitigation of signals fromthe neighboring interfering cells at the wireless device receiverenhances downlink reception quality. For example, reception of the datachannel and/or control channels (e.g., Physical Downlink Shared Channel(PDSCH), Physical Downlink Control Channel (PDCCH), Physical HybridAutomatic Repeat Request Indicator Channel (PHICH), Physical ControlFormat Indicator (PCFICH), etc.) at the wireless device can be enhancedif the interference caused by signals (e.g., CRS, etc.) transmitted bythe interfering cells on these channels can be partly or fullymitigated. Due to network planning in homogeneous networks, the CRS donot collide (i.e., radio resources used for CRSs do not overlap) betweenthe closest neighbor cells. Therefore, typically in such a plannedhomogeneous network, some of the REs of the data channel and/or controlchannels in a serving cell of the wireless device will collide with theREs containing CRS in one or a few strongest neighbor/interfering cells.It is therefore beneficial to cancel interference resulting from thetransmission of CRS or any type of reference signal in such interferingcells on REs that overlap with the data or control channels in theserving cell.

However, even when CRS in neighboring cells do not collide, substantialperformance improvement is achieved when the interference cancellationis applied only in a certain network configuration(s) and loadingsituation(s). Furthermore, in a homogeneous network or in any type ofnetwork deployment where no resource partitioning between victim andaggressor cells is used, the wireless device will typically have tocontinuously cancel interference in all subframes. Even in aheterogeneous network, the larger part of the network is homogeneous(i.e., includes a single layer cell layout). Therefore, the drawback ofcontinuous interference cancellation is the impact on the powerconsumption, processing, memory, and complexity in general at thewireless device.

Normally, IC is used without taking into account all importantconditions under which both performance gain is achieved and without anyregard to the impact on wireless device power consumption andcomplexity. However, as discussed below, systems and methods aredisclosed herein for triggering IC, or mitigation, at the wirelessdevice in response to certain conditions such as, e.g., the traffic loadin the aggressor cells and relation of signal characteristics betweenthe serving and aggressor cells. This in turn leads to more accurateinterference measurement for deriving channel quality (e.g., CQI),resulting in higher user throughputs.

In this regard, FIG. 1 illustrates one example of a cellularcommunications network 10 operating according to one embodiment of thepresent disclosure. As illustrated, the cellular communications network10 includes a number of base stations 12-1, 12-2, and 12-3 and awireless device 14. Note that while only three base stations 12 and onewireless device 14 are illustrated in the example of FIG. 1, thecellular communications network 10 may include any number of basestations 12 and wireless devices 14. Note that while the description ofsome of the embodiments below uses 3GPP LTE terminology (e.g., CRS,IC-CRS, etc.), the embodiments described herein can be used in anysuitable type of cellular network. Further, while some of theembodiments relate to interference resulting from transmission of CRS,the embodiments are equally applicable to other types of reference orpilot signals.

In one embodiment, the wireless device 14 is connected to a cell servedby the base station 12-1. In this case, the cell served by the basestation 12-1 is referred to as a serving cell of the wireless device 14.In another embodiment, the wireless device 14 is performing measurementson the cell served by the base station 12-1, in which case the cell isreferred to as a measured cell of the wireless device 14. Thus, the cellserved by the base station 12-1 is sometimes referred to more generallyas a desired cell of the wireless device 14 in order to cover both ofthe aforementioned embodiments. As such, as used herein, a desired cellis a serving cell of the wireless device 14, a measured cell of thewireless device 14, or any other cell from which a received signal atthe wireless device 14 is interfered with by transmissions from theinterfering cell(s).

Cells served by the base stations 12-2 and 12-3 are interfering, oraggressor cells, in that transmissions by the base stations 12-2 and12-3 result in interference during reception of the downlink for thedesired cell at the wireless device 14. More specifically, thetransmission of reference signals (e.g., CRSs), which are generallytransmitted at a maximum transmit power level, by the base stations 12-2and 12-3 for the interfering cells results in downlink interference atthe wireless device 14 for the downlink from the base station 12-1 forthe desired cell. Radio resources (e.g., REs) utilized for thetransmission of the reference signals by the base stations 12-2 and 12-3for the interfering cells may overlap or coincide with the radioresources utilized for the transmission of the reference signal by thebase station 12-1 for the desired cell. In this case, the referencesignals of the desired and interfering cells are referred to herein as“colliding” or “non-shifted” reference signals. Conversely, radioresources (e.g., REs) utilized for the transmission of the referencesignals by the base stations 12-2 and 12-3 for the interfering cells maynot overlap or not coincide (partially or completely) the radioresources utilized for the transmission of the reference signal by thebase station 12-1 for the desired cell. In this case, the referencesignals of the desired and interfering cells are referred to herein as“non-colliding” or “shifted” reference signals. Notably, as used herein,colliding reference signals are reference signals that fully or, in someembodiments, partly overlap in time and/or frequency. Conversely,non-colliding reference signals are reference signals that do notoverlap in time and/or frequency.

FIGS. 2A and 2B illustrate one example of colliding reference signalsand non-colliding reference signals, respectively. In this example, thereference signals are CRSs. In LTE, a CRS consists of a number ofreference symbols, which are referred to herein as CRS symbols, ofpredefined values inserted within the first and third last OrthogonalFrequency Division Multiplexing (OFDM) symbol of each slot and with afrequency-domain spacing of six subcarriers. Furthermore, there is afrequency-domain staggering of three subcarriers for CRS symbols withinthe third last OFDM symbol. LTE defines six possible frequency shifts ofthe CRS symbols. In the example of FIG. 2A, both the serving/measuredcell and the interfering cell (e.g., the cell of the base station 12-2)use a frequency shift of zero. As a result, CRS transmissions for theserving/measured cell and the interfering cell use the same REs and, assuch, the CRSs are colliding. In contrast, in the example of FIG. 2B,the serving/measured cell uses a frequency shift for CRS of zero,whereas the interfering cell uses a frequency shift for CRS of two. As aresult, CRS transmission for the serving/measured cell and theinterfering cell use different REs and, as such, the CRSs arenon-colliding.

Importantly, the wireless device 14 is capable of performing inter-cellinterference mitigation using one or more inter-cell interferencemitigation techniques (e.g., IC-CRS technique(s)) to mitigate inter-cellinterference at a receiver of the wireless device 14 while the wirelessdevice 14 receives a signal (e.g., a channel, a physical signal,performs a measurement, etc.) transmitted by the base station 12-1 forthe desired cell. This inter-cell interference is caused by transmissionof signals (e.g., reference signals) by the base stations 12-2 and 12-3for the interfering cells. Notably, while only one serving/measured cellis described herein for ease of discussion, the wireless device 14 mayperform interference mitigation for multiple serving/measured cells in amulti-carrier or Carrier Aggregation (CA) scheme. For example, in amulti-carrier scenario, embodiments disclosed herein are applicable tothe reception of signals from each serving cell of the wireless device14.

In order to perform inter-cell interference mitigation, the wirelessdevice 14 must estimate the interference from the interfering cells. InLTE, the interference is estimated on the CRS transmitted in thedownlink from the serving/measured cell. However, as discussed above,the CRSs from the interfering cells may or may not collide with the CRSof the serving/measured cell. For the colliding (or non-shifted) CRSscenario, the measured interference is based only on the CRStransmissions of the interfering cells. Since CRSs are transmittedwithin full power, the wireless device 14 will measure “high”interference independently of the traffic load in the interfering cells.Thus, such interference estimates are only accurate in scenarios wherethe traffic level in the interfering cells is high. Otherwise, suchinterference estimates overestimate the amount of interference.Conversely, in the non-colliding (or shifted) scenario, the measuredinterference is based on data transmissions from the interfering cellsand, as such, the measured interference depends on the traffic level, orload, in the interfering cells. However, such interference estimateswould underestimate interference in that they do not capture the impactof the shifted CRS transmissions of the interfering cells.

In order to address these issues, as discussed below, interferencemitigation at the wireless device 14 is triggered only under certainconditions, e.g., non-colliding reference signals and high traffic levelin the interfering cell(s). More specifically, interference mitigationat the wireless device 14 is triggered by first assessing one or moreconditions under which the wireless device 14 should performinterference mitigation of at least a certain type of radio signals(e.g., CRS) received at its receiver from the interfering cell(s). Theconditions are based on information related to, in some embodiments, theinterfering cells, the serving/measured cell, the wireless device 14,and/or a relation between any combination of the wireless device 14, theinterfering cells, and the serving/measured cell. When the condition(s)are satisfied, interference mitigation at the wireless device 14 istriggered. In one embodiment, the determination to trigger interferencemitigation is made by a network node (e.g., the base station 12-1 of theserving/measured cell), and interference mitigation at the wirelessdevice 14 is triggered by signaling, either explicitly or implicitly, anindication to the wireless device 14 enabling the wireless device 14 toinitiate interference mitigation of radio signals (e.g., IC-CRS) fromone or more of the interfering cell(s), as determined by assessing thecondition(s). As a result, more accurate interference estimates areobtained for, e.g., deriving channel quality (e.g., CQI), which in turnresults in higher user throughputs.

Before proceeding, it should be noted that the systems and methodsdisclosed herein are particularly applicable to scenarios when there isno resource partitioning between the serving/measured cell andinterfering cell(s), e.g., when no low interference subframes or AlmostBlank Subframes (ABSs) are configured or used in the interferingcell(s). Typically in a homogeneous network deployment, there is noresource portioning between the serving cell and interfering cell(s).However, there is no resource partitioning between the serving cell andinterfering cell(s) even in some heterogeneous network configurations(e.g. when the serving cell is served by a High Power Node (HPN) (e.g.,a macro BS) and the interfering cell(s) is served by a Low Power Node(LPN) (e.g., a micro, pico, or femto BS)).

FIG. 3 is a flow chart that illustrates the operation of a nodeassociated with the cellular communications network 10 to triggerinterference mitigation at the wireless device 14 according to oneembodiment of the present disclosure. This node may be a network node(e.g., a radio network node such as the base station 12-1 of theserving/measured cell) or the wireless device 14. Notably, while the“steps” of FIG. 3 are illustrated in a particular order, the “steps” maybe performed in any desired order (or even concurrently) depending onthe particular implementation, unless a specific ordering is explicitlyor implicitly required. The same is true for all flow charts and similardiagrams included herein.

As illustrated, the node obtains information that is indicative of asignal load or interference level in at least one interfering cell (step100). In the example of FIG. 1, there are two interfering cells.However, there may be any number of one or more interfering cells. Thisinformation may be, for example, any metric that can depict the load ofa transmitted signal in the interfering cells and/or signal qualityreceived at the wireless device 14 from the interfering cells. Theinterference level of an interfering cell means the interferenceexperienced by the wireless device 14 at the receiver of the wirelessdevice 14 is caused by the interfering cell. The node also obtainsinformation that is indicative of a signal relation between referencesignals used in the serving/measured cell and the interfering cells(step 102). In one embodiment, the signal relation is either collidingor non-colliding. For example, in one embodiment, the reference signalsare CRS, and the signal relation is either colliding or non-collidingCRSs.

Still further, the node obtains information that is indicative of atiming relation between signals transmitted by the base station 12-1 forthe serving/measured cell and by the base stations 12-2 and 12-3 for theinterfering cells and/or a timing relation between signals received atthe wireless device 14 from the serving/measured cell and theinterfering cells (step 104). In one example, the timing relation isdefined as a maximum absolute deviation in frame start timing betweenany pair of desired and interfering cells that have overlapping coverageareas. Note that while steps 100-104 are all performed in this example,the present disclosure is not limited thereto. In some embodiments, allthree of these steps may not be performed (e.g., only steps 100 and 102may be performed or only steps 100 and 104 may be performed or onlysteps 102 and 104 may be performed).

Optionally, in some embodiments, the node also obtains supplementalinformation (step 106). The supplemental information may, in someembodiments, include one or more of the following: information that isindicative of a signal operation type at the wireless device 14 (e.g.,channel estimation, interference estimation, etc.), information that isindicative of a battery life of the wireless device 14, information thatis indicative of power consumption of the wireless device 14,information that is indicative of a network deployment scenarioexperienced by the wireless device 14 (e.g., homogeneous orheterogeneous), information that is indicative of a location, orposition, of the wireless device 14 with respect to the interferingcells, and information that is indicative of a frequency error betweenthe serving/measured cell and the interfering cells.

The node determines whether to trigger interference mitigation at thewireless device 14 based on one or more predefined criteria and at leastsome of the information obtained in steps 100-106 (step 108). In otherwords, the node accesses one or more conditions under which the wirelessdevice 14 should perform interference mitigation of at least a certaintype(s) of radio signals (e.g., CRS) received at the receiver of thewireless device 14 from the interfering cells, as defined by the one ormore predefined criteria, based on the information. In one embodiment,the node determines that interference mitigation is to be triggered onlyif all of the predefined criteria are satisfied. More specifically, inone embodiment, the predefined criteria, or in other words theconditions, for triggering interference mitigation at the wirelessdevice 14 include one or more criteria based on any one or a combinationof (and in one embodiment all of): an interference mitigation capabilityof the wireless device 14, a signal load or interference level in theinterfering cell(s), a signal relation between the desired cell and theinterfering cell(s), and a timing relation between the desired cell andthe interfering cell(s).

If the node determines that interference mitigation is not to betriggered, the process ends. Optionally, the process may be repeated asdesired. If the node determines that interference mitigation is to betriggered at the wireless device 14, the node then triggers interferencemitigation at the wireless device 14 (step 110). Notably, depending onthe particular embodiment, interference mitigation may be triggered forall of the interfering cells or only the interfering cell(s) for whichthe predefined criteria are satisfied. The manner in which the nodetriggers the interference mitigation can vary depending on theparticular embodiment. More specifically, in one embodiment, the node isthe wireless device 14, and the wireless device 14 triggers interferencemitigation locally at the wireless device 14. In another embodiment, thenode is a network node, and the network node triggers interferencemitigation at the wireless device 14 by either implicit or explicitsignaling to the wireless device 14. This signaling indicates to thewireless device 14 that the wireless device 14 is to activateinterference mitigation, at least with respect to one or more identifiedinterfering cells.

An example of implicit signaling is an existing Information Element (IE)in 3GPP LTE called “CRS assistance information.” The CRS assistanceinformation contains information about the interfering cells, asdescribed above. In some embodiments, the wireless device 14 performsIC-CRS or interference mitigation of specific signals provided that thewireless device 14 is configured with at least the information about theinterfering cells from the serving cell of the wireless device 14 (e.g.,configured via the CRS assistance information). For instance, a standardor configuration may require the wireless device 14 to perform IC-CRS orinterference mitigation of specific signals provided that the wirelessdevice 14 is configured with at least the information about theinterfering cells from the serving cell of the wireless device 14 (e.g.,configured via the CRS assistance information). As another example, itmay additionally or alternatively be predefined that the wireless device14 shall meet one or more sets of predefined requirements provided thatthe wireless device 14 receives at least the information about theinterfering cells from the serving cell (e.g., CRS assistanceinformation). Examples of predefined requirements are UE performancerequirements of data (e.g., PDSCH) and/or control channels (e.g.PDCCH/PHICH/PCFICH), Channel State Information (CSI) measurements (e.g.,CQI), etc. Thus, in this example, the network node may implicitlytrigger interference mitigation at the wireless device 14 by signalingcorresponding CRS assistance information to the wireless device 14.

An example of explicit indication is an indicator that indicates thatthe wireless device 14 is to activate interference mitigation. Forexample, in its simplest form, the explicit indication can be expressedin terms of a Boolean or binary parameter, e.g., with two levels such as0 and 1 representing deactivate interference mitigation and activateinterference mitigation, respectively. The indication may also indicatewhether interference mitigation is to be applied to specific type ofsignals or channels (e.g., CRS, Primary Synchronization Signal(PSS)/Secondary Synchronization Signal (SSS), Physical Broadcast Channel(PBCH), etc.) or on all signals or channels of the interfering cells.The indication may also contain additional information such asinformation that informs the wireless device 14 whether the wirelessdevice 14 has to mitigate the interference on all channels of thedesired cell, on a data channel of the desired cell, on control channelsof the desired cell, or only on selected channels (e.g., PDSCH, PDCCH,etc.) of the desired cell. The indication may also contain additionalinformation informing the wireless device 14 of frequency and/or timedomain physical resources on which the wireless device 14 is to performinterference mitigation. For example, a cell can be active in only halfof the available system bandwidth. So, a bitmap showing the usedresources in the frequency domain can be used. In that case, thewireless device 14 can perform interference mitigation only in thecorresponding parts of the frequency domain. The indication may alsocontain information on whether interference mitigation assistedinformation should be used for interference measurements or fordemodulation. In most of the above cases, the explicit indicator may besignaled to the wireless device 14 via a new Radio Resource Control(RRC) signaling.

Now, a detailed discussion of the predefined criteria used to determinewhether to trigger interference mitigation at the wireless device 14 isprovided. The predefined criteria (or conditions) includes one or moreprimary criteria and, in some embodiments, one or more supplementalcriteria. The one or more primary criteria are based on one or more ofthe following: a capability of the wireless device 14 to performinterference mitigation, signal load or interference level in theinterfering cells, signal relation between the desired and interferingcells, and timing relation between the desired and interfering cells,each of which is discussed below.

Interference mitigation capability of the wireless device 14: Theinterference mitigation capability of the wireless device 14 may bedetermined by, for example, a UE category of the wireless device 14.Based on this information, a predefined criterion for triggeringinterference mitigation at the wireless device 14 may be that thewireless device 14 is capable of performing interference mitigation orcapable of performing interference mitigation on a desired type(s) ofspecial signal. If step 108 is performed in a network node, such acriterion would avoid, e.g., transmission of unnecessary interferencemitigation related information to the wireless device 14 if the wirelessdevice 14 is not capable of performing the desired interferencemitigation.

In one embodiment, the node performing the process of FIG. 3 is anetwork node, where the network node may obtain information indicativeof the interference mitigation capability of the wireless device 14 fromthe wireless device 14 or another network node. More specifically, inone embodiment, the wireless device 14 informs the cellularcommunications network 10 (i.e., a network node such as, for example,the base station 12-1) of a type of receiver with which the wirelessdevice 14 is equipped and/or informs the cellular communications network10 if the wireless device 14 has the capability to perform interferencemitigation. The wireless device 14 can further inform the cellularcommunications network 10 of the type(s) of signals on which thewireless device 14 can perform interference mitigation (e.g., CRS, PSS,SSS, etc.). In another embodiment, the node performing the process ofFIG. 3 is the wireless device 14. In this case, the wireless device 14has knowledge of whether it has interference mitigation capability.

Signal load or interference level in the interfering cells: Thisinformation may be, for example, any metric that can depict the load ofa transmitted signal in the interfering cells and/or signal qualityreceived at the wireless device 14 from the interfering cells. Theinterference level of an interfering cell means the interferenceexperienced by the wireless device 14 at the receiver of the wirelessdevice 14 is caused by the interfering cell. Examples of metrics orinformation that are indicative of the signal load or interference levelin an interfering cell include: radio node transmit power (e.g., atransmit power of the base station 12-2/12-3 serving the interferingcell), received signal quality at the wireless device 14 with respect tothe interfering cell (e.g., Reference Signal Received Quality (RSRQ)measured on interfering cell), utilization of radio resources at theradio node (e.g., base station) serving the interfering cell (e.g.,usage of physical Resource Blocks (RBs), REs, etc.), transmissionintensity of data (e.g., PDSCH) and/or control channel (e.g., PDCCH,PHICH, etc.) from the interfering cell.

The predefined criteria for triggering interference mitigation at thewireless device 14 for a particular interfering cell may include one ormore criteria based on the signal load and/or interfering level in theinterfering cells. For example, the predefined criteria may include acriterion that interference mitigation at the wireless device 14 is tobe triggered for the interfering cell if the signal load or interferencelevel in the interfering cell if the signal load or interference levelfor the interfering cell meets a defined condition, e.g., is belowcertain threshold. This threshold may be indicative of a low or mediumload, or traffic level, in the interfering cell. This criterion may, inone embodiment, be combined with a criterion that the reference signalsof the desired and interfering cells be non-colliding. As one particularexample, the criterion may be that interference mitigation at thewireless device 14 is to be triggered for the interfering cell if thereference signals are non-colliding and the average resource utilizationin the interfering cell is below 30%, i.e., only 30% of the PhysicalResource Blocks (PRBs) in the downlink for the interfering cell are usedon average. This condition may be checked for all interfering cells,e.g. N strongest interfering cells of the wireless device 14. In oneembodiment, interference mitigation is triggered only if all of the Nstrongest interfering cells satisfy this condition (as well as any otherconditions). In another embodiment, interference mitigation is triggeredon a per interfering cell basis such that interference mitigation for aparticular interfering cell is triggered if the condition(s) (i.e., thepredefined criteria) are satisfied for that interfering cell.

The information that is indicative of the signal load or interferencelevel in the interfering cells may be obtained by the node in anysuitable manner. In one embodiment, the node is a network node, and thenetwork node obtains the aforementioned information from information ormeasurements received from the interfering cell and/or from the wirelessdevice 14 (e.g., RSRQ). In another embodiment, the node is the wirelessdevice 14, and the wireless device 14 obtains the aforementionedinformation by, for example, obtaining a signal quality (e.g., RSRQ,Signal-to-Interference plus Noise Ratio (SINR), etc.) of the signalsreceived from the interfering cells. As another example, the wirelessdevice 14 may receive explicit information from a serving cell of thewireless device 14 regarding the signal load of the interfering cell(s)(e.g., average transmit power). The signal load or interference levelcan be estimated over a certain time period, e.g. 200 Milliseconds (ms).

Signal relation between the desired cell and the interfering cells: Theone or more predefined criteria may additionally or alternativelyinclude at least one criterion that is based on the signal relationbetween the desired cell and the interfering cells. This signal relationrefers to the relation between time-frequency locations of the referencesignals used by the desired cell and the interfering cell(s). An exampleis whether the CRS transmitted by the desired cell and the interferingcell are colliding or non-colliding. The collision of the CRS occurswhen the REs containing the CRS in the desired and interfering cellsoverlap in both time and frequency. The collision of the CRS can beavoided by shifting the CRS in the frequency domain between the desiredand the interfering cells. This is done during network planning and istherefore not changed frequently. The node therefore checks whether theCRS are colliding or not between the desired and interfering cells ofthe wireless device 14.

In one example, the predefined criteria include a criterion thatinterference mitigation at the wireless device 14 for an interferingcell is to be triggered when the CRS of the desired cell and theinterfering cell are not colliding. This may be particularly beneficialwhen it is desired to reduce interference on the downlink data and/orcontrol channels at the wireless device 14. As another example, if it isdesirable to enhance channel estimation (e.g., based on CRS) and/orenhance CSI performance (e.g., also based on CRS) at the wireless device14, then the predefined criteria may include a criterion thatinterference mitigation is to be triggered at the wireless device 14 foran interfering cell if the CRS for the desired cell and the interferingcell are colliding. Typically, it is more desirable to mitigate theinterference on the data and/or control channels especially at lowloads, or low traffic levels, in the interfering cell(s) due to thelarge impact of CRS transmissions in the interfering cell(s) for theshifted CRS scenario. Therefore, interference mitigation may betriggered more commonly, or frequently, at the wireless device 14 whenthe CRSs do not collide.

Note that, in 3GPP LTE, the colliding and non-colliding CRS can beexpressed mathematically using a mod x operation. In case of a singleantenna in a base station, there are six possible frequency shifts toavoid CRS collision. As an example, if CRS used in cell 1 and cell 2collide, then the relation between their Physical Cell Identities (PCIs)#1 and #2 can be expressed as a mod 6 operation. In another example, ifCRS used in cell 3 and cell 4 do not collide, then the relation betweentheir Pas #3 and #4 can also be expressed as a mod 6 operation. Theseexamples are expressed mathematically below:

-   -   Colliding CRS between cell 1 and cell 2: (PCIcell1−PCIcell2)mod        6=0; and    -   Non-colliding CRS between cell 3 and cell 4:        (PCIcell3−PCIcell4)mod 6 !=0.

In one embodiment, the node is a network node, and the network nodeobtains information that is indicative of the signal relation betweenthe reference signals of the desired and interfering cell(s) fromanother network node, the wireless device 14, or a combination thereof.For example, the network node may obtain the Pas of the desired andinterfering cell(s) and, based on the PCIs, determine whether thereference signals of the desired and interfering cell(s) are collidingor non-colliding using, e.g., a mod x operation, as described above. Inanother embodiment, the node is the wireless device 14, and the wirelessdevice 14 obtains information that is indicative of the signal relationbetween the reference signals of the desired and interfering cell(s)from one or more network node (e.g., the base stations 12-1, 12-2, and12-3 of the serving and interfering cells, respectively). For example,the wireless device 14 may obtain the Pas of the desired and interferingcell(s) during synchronization or cell search and, based on the PCIs,determine whether the reference signals of the desired and interferingcell(s) are colliding or non-colliding using, e.g., a mod x operation,as described above.

Timing relation between the desired and interfering cells: The one ormore predefined criteria may additionally or alternatively include atleast one criterion that is based on a timing relation between thedesired and interfering cell(s). This timing refers to a timing relationbetween signals transmitted by the desired cell and the interferingcell(s) and/or a timing relation between signals received at thewireless device 14 from the desired cell and the interfering cell(s).This timing relation is also interchangeably referred to as a transmittime synchronization or transmit time alignment between the signalstransmitted by the desired cell and the interfering cell(s). This timingrelation is also interchangeably referred to as a cell phasesynchronization accuracy. In one example, the timing relation is definedas a maximum absolute deviation in frame start timing between any pairof desired and interfering cells that have overlapping coverage areas.Typically, the transmit time alignment between cells can be in the orderof 1-10 Microseconds (μs).

Typically, the wireless device 14 has a single receiver (e.g., a singleInverse Fast Fourier Transform (IFFT)/Fast Fourier Transform (FFT)).Therefore, the wireless device 14 can perform interference mitigation ofsignals received from the interfering cell(s) provided these interferingsignals and signals of the desired cell arrive at the receiver of thewireless device 14 well within the Cyclic Prefix (CP) length (e.g., 4.7μs for normal CP). Therefore, the receive time difference of signals atthe wireless device 14 should be within 1-2 μs. Thus, as one example,the predefined criteria for triggering interference mitigation at thewireless device 14 for an interfering cell may include a criterion thatthe timing relation between the desired cell and interfering cell issuch that the wireless device 14 can perform the interference mitigationusing single receiver (e.g., the timing relation is well within the CPlength at the receiver of the wireless device 14). Otherwise,interference mitigation is not triggered at the wireless device 14. In atypical scenario, the wireless device 14 is in a cell border region ofthe desired cell. In a synchronized homogeneous network, the receivetime difference of signals at the wireless device 14 from the desiredand interfering cells is small since cells are of the same size.Therefore, in such a scenario if cells are synchronized (i.e., thetransmit time is 1-3 μs), then the node can decide to activateinterference mitigation at the wireless device 14.

In one embodiment, the node performing the process of FIG. 3 is anetwork node, and the network node determines the timing relationbetween the cells based on predetermined information, or the networknode can acquire this from another node (e.g., another network node)that has this information. For example, a lookup table mapping thetransmit time relation between a pair of cells in the coverage area andtheir cell identifiers (e.g., cell IDs) can be stored in a network node.The receive time difference of signals received at the wireless device14 from the serving and interfering cells can be determined by thenetwork node explicitly based on a measurement report from the wirelessdevice 14 or implicitly by knowing a location of the wireless device 14.

In the embodiment where the node performing the process of FIG. 3 is anetwork node, the network node can also obtain the receive timedifference measurement of pairs of cells from the wireless device 14.This measurement can be performed by the wireless device 14 for thesignals received at the receiver of the wireless device 14 from thedesired and interfering cell(s). An example of such measurement is aReference Signal Time Difference (RSTD) measurement performed on aPositioning Reference Signal (PRS) by the wireless device 14 forObserved Time Difference of Arrival (OTDOA) positioning. Thismeasurement report can more accurately depict whether the wirelessdevice 14 can perform the interference mitigation of signals from acertain interfering cell or not. Therefore, the network node can usethese measurement results and/or also transmit time difference at theradio nodes to decide whether to trigger interference mitigation at thewireless device 14 and also to decide if the interfering cells are to beincluded in assistance information sent to the wireless device 14 forinterference mitigation. The network node may even take into account theradio channel characteristics (e.g., multipath delay profile) todetermine or predict expected spread of signals from the desired andinterfering cells at the receiver of the wireless device 14. If thechannel has a very high delay spread (e.g., 2 μs), then the network nodemay decide not to trigger interference mitigation at the wireless device14 unless the desired and interfering cells are of small size, e.g.,100-200 meters. The radio channel characteristics can be determined onsignals sent by the wireless device 14 in the uplink for the servingcell of the wireless device 14.

In another embodiment, the node performing the process of FIG. 3 is thewireless device 14, and the wireless device 14 determines the timing ofeach cell during synchronization or during cell search. Based on thedetermined timing, the wireless device 14 determines the timing relationbetween the signals received at its receiver from the desired andinterfering cells. The wireless device 14 may also determine the CPlength of the desired and interfering cells during cell search.

As discussed above, in some embodiments, the predefined criteria (orconditions) include one or more supplemental criteria in addition to theone or more primary criteria. The one or more supplemental criteria arebased on one or more of the following: signal operation type of thewireless device 14, battery life of the wireless device 14, powerconsumption of the wireless device 14, network deployment scenario,location of the wireless device 14 with respect to the interferingcells, and frequency error between the desired and interfering cells,each of which is discussed below.

Signal operation type: Examples of the signal operation type of thewireless device 14 include channel estimation, interference estimation,demodulation, and CSI (e.g., CQI) assessment. As discussed below, thepredefined criteria for triggering interference mitigation at thewireless device 14 may include one or more criterion based on the signaloperation type at the wireless device 14. For example, if the signaloperation type is interference estimation, the node may triggerinterference mitigation at the wireless device 14 for only inactiveinterfering cell(s) prior interference estimation. In contrast, the nodemay trigger interference mitigation for both active and inactiveinterfering cells prior to channel estimation. If the node performingthe process of FIG. 3 is the wireless device 14, the wireless device 14may use one or more predefined criteria based on signal operation typeto perform interference mitigation of signals only from suitableinterfering cells, i.e., which will lead to improved reception qualityof signals from the desired cell.

In one embodiment, the node performing the process of FIG. 3 is anetwork node. The network node can determine the signal operation typeof the wireless device 14, whether it uses CRS or some other type ofreference signals (e.g., Demodulation Reference Signal (DMRS), ChannelState Information—Reference Symbol (CSI-RS), etc.) for channelestimation and/or interference estimation, based on, e.g., one or moreof the following:

-   -   Predefined behavior of the wireless device 14 such as, e.g.,        using a certain type of reference signal for channel estimation        as defined by a standard.    -   Predefined requirements for the wireless device 14, e.g.,        wireless device PDSCH demodulation requirements that require the        wireless device 14 to use a certain type of reference signal for        channel estimation are specified for a certain antenna        transmission mode in a corresponding standard (e.g., a new LTE        standard).    -   Antenna transmission mode. For example, at least transmission        modes 1-8 (e.g., transmit diversity, spatial diversity, closed        loop scheme, etc.) use CRS for channel and/or interference        estimation. Since the cellular communications network 10        configures the wireless device 14 with a certain antenna mode,        the network node knows (or can know) the antenna mode currently        used by the wireless device 14.    -   Explicit indication from the wireless device 14 such as, for        example, an explicit indication from the wireless device 14 that        the wireless device 14 uses CRS for channel estimation.        In another embodiment, the node performing the process of FIG. 3        is the wireless device 14. The wireless device 14 performs        interference mitigation and channel estimation. Therefore, the        wireless device 14 can retrieve this information locally (e.g.,        from its own processing unit).

Battery life or power consumption: In a homogeneous network, thewireless device 14 is expected to perform interference mitigation tomitigate the interference caused by the interfering signals from theinterfering cells continuously, at least most of time, or whenever thewireless device 14 is served or performs measurements. This is becausetraffic load in a homogeneous network is much higher than that in aheterogeneous network. Even in a heterogeneous network, in principle,the wireless device 14 is required to monitor the reception of downlinkchannels (e.g., PDCCH) in all subframes. However, in a heterogeneousnetwork, the wireless device 14 performance requirements (e.g., PDCCH,PDSCH reception performance, etc.) are defined in subframes whichoverlap with ABS in interfering cell(s). However, in a homogeneousnetwork, the wireless device 14 has to listen to the control channel(e.g., PDCCH monitoring) in all downlink subframes of the serving celland can therefore be served in all downlink subframes in a radio frameof the serving cell. The interference mitigation requires more powerconsumption, memory, and processing at the wireless device 14.

Therefore, in one embodiment, the predefined criteria for triggeringinterference mitigation at the wireless device 14 include one or morecriteria based on the battery life and/or power consumption at thewireless device 14. The one or more criteria may be defined trigger ornot trigger interference mitigation at the wireless device 14 in such amanner as to avoid battery drainage at the wireless device 14. Forexample if the battery life at the wireless device 14 is below athreshold, then the node may not trigger interference mitigation at thewireless device 14.

In one embodiment, the node performing the process of FIG. 3 is anetwork node, and the network node can obtain information indicative ofthe battery life of the wireless device 14 by, for example, explicitlyreceiving a current, or present, battery level (e.g., absolute value interms of watts, discrete power levels such as low, medium, and high,etc.) from the wireless device 14. The network node can also implicitlydetermine a state of the battery life of the wireless device 14 by, forexample, observing an activity level of the wireless device 14 over apast time period (TO). For example, if the wireless device 14 isreceiving data over the last certain number of frames (e.g., 100-200),then the network node may implicitly assume that the battery life of thewireless device 14 is low.

In another embodiment, the node performing the process of FIG. 3 is thewireless device 14. The wireless device 14 can explicitly determine thecurrent battery life of the wireless device 14 and/or estimate anexpected power consumption at the wireless device 14 if interferencemitigation is triggered. In one example, the wireless device 14 maydecide to trigger interference mitigation only if battery life at thewireless device 14 is above a threshold.

Network deployment scenario: The network deployment scenariocharacterizes whether the desired and interfering cells belong to ahomogeneous network or a heterogeneous network. For example, if thedesired and interfering cells are of the same type (e.g., macro cells),then the network deployment scenario is assumed to be homogeneous.Otherwise, the network deployment scenario is assumed to beheterogeneous. The cell type is characterized by one or more attributessuch as power class of the radio network node serving the cell, cellsize (e.g., cell radius, inter-site distance between cells, cell range,etc.), etc.

In one embodiment, the predefined criteria for triggering interferencemitigation at the wireless device 14 include one or more criterion basedon the network deployment scenario. For example, a criterion may bedefined such that the node triggers interference mitigation providedthat the desired cell and the interfering cell(s) are homogenous (e.g.,provided the cells are macro cells or the cells are pico cells). Asanother example, a criterion may be defined such that the node triggersinterference mitigation at the wireless device 14 in some heterogeneousnetwork scenarios such as, e.g., when the desired cell is served by aHPN (e.g., a macro cell) and the interfering cell(s) is served by an LPN(e.g., a pico cell). In this case, no low interference subframes (e.g.,ABS) are needed in the interfering cells.

In one embodiment, the node performing the process of FIG. 3 is anetwork node. The network node can determine the network deploymentscenario of the desired and interfering cells based on, e.g.,predetermined information or information received from another node(e.g., O&M, OSS, SON, etc.).

In another embodiment, the node performing the process of FIG. 3 is thewireless device 14, and the wireless device 14 can determine the celltype (e.g., macro, pico, etc.) of the desired and interfering cellsbased on any of: radio measurements, acquisition of system parameters, apriori or historical knowledge, etc. In large cells, when in a cellborder area, the signal level (e.g., a Path Loss (PL) measurement suchas, e.g., PL=difference in Decibels (dB) between CRS Transmit (Tx) power(Decibel-Milliwatt (dBm))—Reference Signal Received Power (RSRP) (dBm))is lower than that in small cells. The wireless device 14 may also readthe system information of a cell to determine the value of the parameterthat indicates a transmit power of certain signals, e.g., CRS. In largecells, the CRS transmit power is larger than that in a small cell. Thewireless device 14 may also store cell type information obtained in thepast. The wireless device 14 may decide to trigger interferencemitigation when the desired and interfering cells are of certain types.In one example, the wireless device 14 may trigger interferencemitigation when the desired and interfering cells are homogeneous (e.g.,all are macro cells or all are pico cells). In another example, thewireless device 14 may trigger interference mitigation when the desiredand interfering cells are served by a HPN and an LPN(s), respectively.In another example, the wireless device 14 may not trigger interferencemitigation when the desired and interfering cells are served by an LPNand a HPN(s), respectively.

Location of the wireless device 14: The one or more predefined criteriafor triggering interference mitigation at the wireless device 14 mayalso include one or more criterion based on the location of the wirelessdevice 14 with respect to the interfering cells and/or the desired cell.Such a criterion enables the node to determine the severity of theinterfering signals at the wireless device 14 received from theinterfering cells. For example, if the wireless device 14 is in the cellborder region of the desired cell, then the node may infer that thewireless device 14 is more severely affected by the interference fromthe interfering cells. One or more criterion may be defined such thatthe node triggers interference mitigation at the wireless device 14 inthis case. The location of the wireless device 14 can be expressed interms of, e.g., a geometry factor (e.g., a ratio of desired cellreceived power to interference), desired cell SINR, etc.

In one embodiment, the node performing the process of FIG. 3 is anetwork node, and the network node may determine the location of thewireless device 14 based on, e.g., one or more of the following: radiomeasurements performed by the wireless device 14 and location orpositioning of the wireless device 14, e.g., based on positioningmethods such as Enhanced Cell Identity (E-CID) measurements such as UEReceive (Rx)-Tx time difference, Assisted Global Navigation SatelliteSystem (A-GNSS), OTDOA, etc. In another embodiment, the node performingthe process of FIG. 3 is the wireless device 14, and the wireless device14 can determine its location based on, e.g., a suitable radiomeasurement (e.g., relative RSRP between the desired and interferingcells), a suitable positioning method, etc. One or more criterion may bedefined such that the wireless device 14 triggers interferencemitigation, e.g., only if the wireless device 14 is in a cell borderregion (which is also known as the Cell Range Expansion (CRE)) of thedesired cell. In this case, interference mitigation will enhanceperformance of the wireless device 14.

Frequency error between the desired and interfering cells: Thepredefined rules for triggering interference mitigation at the wirelessdevice 14 may also include one or more criterion based on the frequencyerror between the desired and interfering cells. For example, the nodemay decide to trigger interference mitigation at the wireless device 14provided that the frequency error between the desired and interferingcells is below a threshold, e.g., a maximum difference threshold of 200Hertz (Hz). In one embodiment, the node performing the process of FIG. 3is a network node, and the network node can determine the relativefrequency error (i.e., between desired and interfering cells) based on,e.g., cell type or base station power class of the corresponding basestations 12 since the frequency error depends upon the base stationpower class. In another embodiment, the node performing the process ofFIG. 3 is the wireless device 14, and the wireless device 14 candetermine the difference in frequencies between the cells, e.g., duringthe synchronization procedure.

Note that in describing the various example embodiments disclosedherein, the description may refer to certain conditions, considerations,or criteria as “minimum,” “necessary,” “mandatory,” or the like, and toothers as “supplemental,” “optional,” “additional,” or the like.However, any suitable consideration or criterion, including any of theexamples identified above, may be mandatory or optional in a particularimplementation, and various embodiments of the present disclosure mayutilize any appropriate combination of considerations or criteria.

As discussed above, in some embodiments, triggering interferencemitigation at the wireless device 14 may be based, at least in part, onthe signal operation type at the wireless device 14. Two examples of thesignal operation type are interference estimation and channelestimation. As discussed below in detail, in one embodiment,interference mitigation is triggered conditionally for interferenceestimation and channel estimation. However, before describing thisembodiment, a brief discussion of two examples of unconditionalinterference mitigation at a wireless device when performinginterference estimation and channel estimation is provided. The firstexample is illustrated in FIG. 4A where a signal from a desired cell isreceived at the wireless device 14, Interference Mitigation (IM) isunconditionally performed on special REs (e.g., REs carrying CRSsymbols) for both active and inactive interfering cells, and theninterference and channel estimation are performed on the special REsafter interference mitigation. An active interfering cell is one that istransmitting data, whereas an inactive interfering cell is on that isnot transmitting data. One issue with this approach is that interferencemay be overestimated, particularly if one or more of the interferingcells are inactive. In the example of FIG. 4B, a signal from a desiredcell is received at the wireless device 14, interference and channelestimation are performed on special REs, and then IM is performed afterinterference and channel estimation. One issue with this approach isthat it is desirable for as much interference to be mitigated aspossible before channel estimation to achieve better, or more accurate,channel estimation.

FIG. 5 illustrates a process for triggering interference mitigation atthe wireless device 14 when performing interference estimation andchannel estimation according to one embodiment of the presentdisclosure. As described below with respect to FIG. 6, in thisembodiment, the wireless device 14 splits interference estimation andchannel estimation into separate steps. Like the process of FIG. 3, theprocess of FIG. 5 is performed by a node associated with the cellularcommunications network 10 (e.g., the base station 12-1 of the desiredcell or the wireless device 14).

As illustrated, the node makes a determination to trigger interferencemitigation at the wireless device 14 for one or more inactiveinterfering cells prior to interference estimation and to triggerinterference mitigation at the wireless device 14 for one or more activeinterfering cells after interference estimation and prior to channelestimation (step 200). The node then triggers interference mitigation atthe wireless device 14 as determined in step 200 (step 202). In thismanner, interference from any inactive interfering cell(s) is(are)mitigated prior to interference estimation, which in turn improves theaccuracy of the interference estimate. In addition, channel estimationis improved by mitigating both active and inactive interfering cellsprior to channel estimation.

FIGS. 6A and 6B illustrate two examples of the process of FIG. 5. In theexample of FIG. 6A, both of the interfering cells are active. As such,any interference cancellation before interference estimation will biasthe interference estimate by underestimating the actual interference.Therefore, according to the process of FIG. 5, interference mitigationat the wireless device 14 is triggered such that: (1) no interferencemitigation is performed at the wireless device 14 prior to interferenceestimation and (2) interference mitigation for both of the interferingcells (which are active) is performed at the wireless device 14 onspecial REs (e.g., REs for CRS) after interference estimation but beforechannel estimation. By doing so, the accuracy of the interferenceestimation is improved.

In contrast, in the example of FIG. 6B, the interfering cell served bythe base station 12-2 (interfering cell 1) is active whereas theinterfering cell served by the base station 12-3 (interfering cell 2) isinactive. In this case, where only interfering cell 1 is active, thewireless device 14 can remove interference from interfering cell 2 andthen perform interference estimation. On the other hand, in case ofchannel estimation it is important that the estimation is done on asignal that is as interference free as possible. Thus in that case, thewireless device can remove interference from the maximum (or differencewith respect to the interference estimation process) number ofinterfering cells. In the above example, if the wireless device 14 iscapable of removing more than one interferer, then the wireless device14 can remove interference from interfering cell 1 and perform channelestimation for the desired cell. Therefore, according to the process ofFIG. 5, in this second example of FIG. 6B, interference mitigation atthe wireless device 14 is triggered such that: (1) interferencemitigation is performed at the wireless device 14 on special REs (e.g.,REs for CRS) for interfering cell 2 prior to interference estimation and(2) interference mitigation is performed at the wireless device 14 onthe special REs (e.g., REs for CRS) for interfering cell 1 (i.e., theresidual interference) after interference estimation but prior tochannel estimation. Since the wireless device 14 removes thecontribution of the non-active interfering cell prior to interferenceestimation, a better match between estimated and actual interference isachieved.

The operation of the wireless device 14 in response to the triggering ofinterference mitigation according to the process of FIG. 5 illustratedin FIG. 7. In particular, FIG. 7 is a functional block diagram thatillustrates the operation of the wireless device 14. As illustrated, thewireless device 14 receives, in this example, a list of activeaggressors (i.e., active interfering cells) from a network node (e.g.,the base station 12-1) (300). Note that while in this example thenetwork node sends the list of active aggressors, the network node maysend a list of aggressor, or interfering, cells that identifies bothactive and inactive interfering cells. As one example alternative, thenetwork node may send separate lists of active and inactive interferingcells, e.g., one list for interference estimation and another list forchannel estimation. In the former case, one list may be sent when thereference signals (e.g., CRS) are non-colliding between the desired andinterfering cells. In the latter case, the two separate lists may besent when the reference signals (e.g., CRS) are colliding between thedesired and interfering cells. Therefore, in one embodiment, the networknode selects between the two alternatives taking into account therelation of the reference signals used between cells and also the typeof operation at the wireless device 14.

The wireless device 14 receives a signal from the base station 12-1 forthe desired cell (302). The wireless device 14 then performsinterference mitigation on special REs (e.g., REs for CRS) for anyinactive aggressor, or interfering, cells as determined from the list ofactive aggressors (304). Then, the wireless device 14 performsinterference estimation on the special REs (306). Next, the wirelessdevice 14 performs interference mitigation on the special REs for anyactive aggressor, or interfering, cells as determined from the list ofactive aggressors (308). Lastly, the wireless device 14 performs channelestimation for the desired cell on the special REs (310).

As discussed above, the processes of FIGS. 3 and 5 may be performed by anetwork node or the wireless device 14. In this regard, FIGS. 8A through8C illustrate three example embodiments of the present disclosure. InFIG. 8A, the process of FIG. 3 is performed by the base station 12-1.More specifically, as illustrated, the base station 12-1 obtains theinformation used for determining whether to trigger interferencemitigation at the wireless device 14 (step 400). The base station 12-1makes a determination to trigger interference mitigation at the wirelessdevice 14 based on the information using one or more predefinedcriteria, or conditions, as described above (step 402). The base station12-1 then triggers interference mitigation at the wireless device 14 inresponse to making the determination of step 402 via explicit orimplicit signaling (step 404). In response, the wireless device 14performs interference mitigation (step 406). While the base station 12-1performs the process of FIG. 3 in this example, in the same manner, thebase station 12-1 may perform the process of FIG. 5.

FIG. 8B illustrates an embodiment in which the process of FIG. 3 isperformed by the wireless device 14. More specifically, as illustrated,the wireless device 14 obtains the information used for determiningwhether to trigger interference mitigation at the wireless device 14(step 500). The wireless device 14 makes a determination to triggerinterference mitigation at the wireless device 14 based on theinformation using one or more predefined criteria, or conditions, asdescribed above (step 502). The wireless device 14 then triggersinterference mitigation at the wireless device 14 in response to makingthe determination of step 502 (step 504). In response, the wirelessdevice 14 performs interference mitigation (step 506). While thewireless device 14 performs the process of FIG. 3 in this example, inthe same manner, the wireless device 14 may perform the process of FIG.5.

Thus, FIG. 8B illustrates an autonomous decision by the wireless device14 to trigger and perform interference mitigation. The autonomousdecision in the wireless device 14 can be governed by one or morepredefined rules, which can be specified in a standard (e.g., a 3GPP LTEstandard). Examples of such rules are as follows. In one example, it maybe predefined that the wireless device 14 shall perform interferencemitigation of signals (e.g., CRS-IC) interfering with reception ofcertain channels (e.g., PDSCH, PDCCH, PCFICH, PHICH, etc.) or whenperforming measurements (e.g., CSI) from the serving/measured cellprovided that a set of the above mentioned conditions are met or atleast when the minimum conditions are met. Such a rule will require thewireless device 14 to verify the predefined conditions and performinterference mitigation provided the conditions are met. This willrequire the wireless device 14 to implement a processing unit to firstverify the conditions and then decide whether to perform interferencemitigation of the interfering signals or not. The conditions and theirlevels or threshold values can also be predefined. For example, it maybe predefined that the wireless device 14 shall perform interferencemitigation (e.g., CRS-IC) provided the following conditions are met,e.g.:

-   -   load in at least one interfering cell is 30% or less,    -   signal quality (e.g., RSRQ, SINR, Signal-to-Noise Ratio (SNR),        etc.) of at least one interfering cell at the wireless device 14        is above a threshold (e.g., −12 dB or above),    -   time offset between cells (i.e., between any pair of desired and        interfering cells) is not more than 2.5 μs and/or that cells are        synchronous, and    -   frequency error between cells (i.e., between any pair of desired        and interfering cells) is not more than 200 Hz, etc.

In another example it may be predefined that the wireless device 14shall meet certain wireless device requirements that are defined basedon interference mitigation of signals (e.g., CRS-IC) which interferewith the reception of certain channels (e.g., PDSCH, PDCCH, PCFICH,PHICH, etc.) from the serving/measured (i.e., desired) cell or whenperforming measurements (e.g., CSI) provided a set of the abovementioned conditions are met or at least when the minimum conditions aremet. The conditions and their levels or threshold values can also bepredefined. This will require the wireless device 14 to implement aprocessing unit to first verify the predefined conditions and thendecide whether to meet the wireless device requirements related to theinterference cancellation or not. To meet these requirements, thewireless device 14 will have to perform the interference cancellation ofthe interfering signals. For example, it may be predefined that thewireless device 14 shall meet certain wireless device requirementsprovided, e.g.:

-   -   load in at least one interfering cell is 30% or less,    -   signal quality (e.g., RSRQ, SINR, SNR, etc.) of at least one        interfering cell at the wireless device 14 is above a threshold        (e.g., −12 dB or above),    -   time offset between cells (i.e., between any pair of desired and        interfering cells) is not more than 2.5 μs and/or that cells are        synchronous, and frequency error between cells (i.e., between        any pair of desired and interfering cells) is not more than 200        Hz, etc.

Examples of wireless device requirements are PDSCH performancerequirements, PDCCH performance requirements, PHICH performancerequirements, PCFICH performance requirements, CSI reportingrequirements (e.g., CQI, Rank Indicator (RI), Precoding Matrix Indicator(PMI) reporting requirements, etc.), etc. The data and control channelUE performance requirements are also interchangeably referred to as UEdemodulation performance, throughput requirements, Block Error Ratio(BLER) performance, reception performance, etc. An example of PDSCHperformance requirements are shown in Table 3 below.

TABLE 3 Minimum Performance Transmit Diversity (FRC); PDSCH performancerequirements Reference Value Propagation Correlation Fraction ofConditions Matrix and Maximum SNR Test Reference OCNG Pattern (Note 1)Antenna Throughput (dB) UE Number Channel Cell 1 Cell 2 Cell 1 Cell 2Configuration (%) (Note 2) Category 1 R.11-4 OP.1 OP.1 EVA5 EVA 5 2x2 705 2-8 FDD FDD FDD Medium (Note 1) The propagation conditions for Cell 1and Cell2 are statistically independent. (Note 2) SNR corresponds to  

 _(s)/N_(oc2) of cell 1.

FIG. 8C illustrates one example of a hybrid embodiment in which anetwork node (which in this example is the base station 12-1) and thewireless device 14 operate together to conditionally triggerinterference mitigation at the wireless device 14. As illustrated, thebase station 12-1 sends an indicator to the wireless device 14 totrigger, or enable, interference mitigation (step 600). The indicatormay be sent, or signaled, in any desired manner (e.g., explicit orimplicit signaling). Further, in one example, the base station 12-1performs the process of FIG. 3 or FIG. 5 to determine whether to sendthe indicator. Before and/or after receiving the indicator, the wirelessdevice 14 obtains the information used for determining whether totrigger interference mitigation at the wireless device 14 (step 602).The wireless device 14 makes a determination to trigger interferencemitigation at the wireless device 14 based on the indicator and theinformation using one or more predefined criteria, or conditions, asdescribed above (step 604). The wireless device 14 then triggersinterference mitigation at the wireless device 14 in response to makingthe determination of step 604 (step 606). In response, the wirelessdevice 14 performs interference mitigation (step 608). While thewireless device 14 performs the process of FIG. 3 in response to theindicator, in the same manner, the wireless device 14 may perform theprocess of FIG. 5 in response to the indicator.

Notably, the process of FIG. 8C can be enforced by defining predefinedrules in a corresponding cellular network standard (e.g., a 3GPP LTEstandard). For example, it may be predefined that the wireless device 14shall meet the UE performance requirements that are based oninterference mitigation (e.g., CRS-IC) provided that the followingconditions are met:

-   -   The wireless device 14 receives at least an explicit indication        to activate interference from the network node;    -   The wireless device 14 receives information about at least one        interfering cell from the network node; and    -   The wireless device 14 meets certain predefined conditions.

FIG. 9 illustrates an embodiment that is similar to that of FIG. 8A butwhere obtaining of at least some of the information used for decidingwhether to trigger interference mitigation at the wireless device 14 isexplicitly shown as being obtained from the wireless device 14 and thebase station 12-2 of one of the interfering cells according to oneembodiment of the present disclosure. As illustrated, the base station12-1 receives IM related information from the wireless device 14 and thebase station 12-2 (steps 700 and 702). In the same manner, the basestation 12-1 may receive IM related information from the base station12-3 of the other interfering cell. The IM related information is any ofthe information described above (e.g., information indicative of theinterference mitigation capability of the wireless device 14,information indicative of a signal load or interference level in theinterfering cell(s), information indicative of a signal relation betweenthe desired cell and the interfering cell(s), etc.). The base station12-1 then processes the information, evaluates the predefined criteriaor conditions for triggering interference mitigation at the wirelessdevice 14, and determines whether to send IM signaling to the wirelessdevice 14 to trigger interference mitigation at the wireless device 14based on the evaluation of the predefined criteria or conditions (step704). In this example, interference mitigation at the wireless device 14is to be triggered and, as such, the base station 12-1 sendscorresponding IM signaling to the wireless device 14 to thereby triggerinterference mitigation at the wireless device 14 (step 706).

FIG. 10 is a block diagram that illustrates functional components of thebase station 12-1, the base station 12-2, and the wireless device 14according to one embodiment of the present disclosure. Each of thefunction components, or modules, may be implemented in hardware,software, or a combination thereof. In this example, the base station12-1 includes a data fusion module 16 that operates to collect theinformation used for deciding whether to trigger interference mitigationat the wireless device 14 and an IM controller 18 that operates toevaluate the one or more predefined criteria or conditions to determinewhether to trigger interference mitigation at the wireless device 14.The data fusion module 16 may collect the information from, e.g., thebase station 12-2 via a backhaul interface between the base stations12-1 and 12-2 (e.g., an X2 interface) and/or the wireless device 14. Thewireless device 14 includes an IM module 20 that operates to performinterference mitigation, as triggered by the IM controller 18 of thebase station 12-1. Note that FIG. 10 is just one example. For instance,the data fusion module 16 and the IM controller 18 may alternatively beimplemented at some other network node or at the wireless device 14.

While the present disclosure is not limited thereto, certainimplementations of the embodiments disclosed herein may provide numerousbenefits including such advantages as: performing load awareinterference estimation, reducing mismatch between actual and estimatedinterference, improving demodulation and CSI (e.g., CQI) estimationperformance, ensuring that UE power consumption is not unnecessarilyincreased, ensuring that UE processing remains within certain limitswhile on average performance is improved, and ensuring that the UEachieves enhanced performance also in a homogeneous network whenoperating in a CRE region.

FIG. 11 illustrates one example of one of the base stations 12 accordingto one embodiment of the present disclosure. This description alsoapplies to other type radio network nodes. Still further, apart from theradio unit 30 and antenna(s) 34, this discussion also applies to networknodes other than radio network nodes. As illustrated, the base station12 includes baseband unit 22 including a processor 24, a memory 26, anda network interface 28 and a radio unit 30 including a transceiver 32coupled to one or more antennas 34. In particular embodiments, some orall of the functionality described above as being provided by the basestation 12 (or similarly a network node) may be provided by theprocessor 24 executing instructions or software stored on acomputer-readable medium, such as the memory 26. For example, either orboth of the data fusion module 16 and the IM controller 18 of FIG. 10may be implemented by the processor 24 executing correspondinginstructions or software stored on a computer-readable medium, such asthe memory 26. Alternative embodiments of the base station 12 mayinclude additional components responsible for providing additionalfunctionality, including any of the functionality identified aboveand/or any functionality necessary to support the embodiments describedabove.

FIG. 12 illustrates one example of the wireless device 14 according toone embodiment of the present disclosure. As illustrated, the wirelessdevice 14 includes a processor 36, memory 38, and a transceiver 40coupled to one or more antennas 42. In particular embodiments, some orall of the functionality described above as being provided by thewireless device 14 may be provided by the processor 36 executinginstructions or software stored on a computer-readable medium, such asthe memory 38. For example, the IM module 20 of FIG. 10 may be providedby the processor 36 executing corresponding instructions or softwarestored on a computer-readable medium, such as the memory 38. Alternativeembodiments of the wireless device 14 may include additional componentsbeyond those shown in FIG. 12 that may be responsible for providingcertain aspects of the wireless device's 14 functionality, including anyof the functionality described above and/or any functionality necessaryto support the embodiments described above.

The following acronyms are used throughout this disclosure.

-   -   3GPP 3^(rd) Generation Partnership Project    -   μs Microsecond    -   ABS Almost Blank Subframe    -   A-GNSS Assisted Global Navigation Satellite System    -   A-GPS Assisted Global Positioning System    -   AoA Angle of Arrival    -   AP Access Point    -   BLER Block Error Ratio    -   BS Base Station    -   BTS Base Transceiver Station    -   CA Carrier Aggregation    -   CC Component Carrier    -   CDMA Code Division Multiple Access    -   CoMP Coordinated Multi-Point    -   CP Cyclic Prefix    -   CPICH Common Pilot Channel    -   CQI Channel Quality Indication/Index    -   CRE Cell Range Expansion    -   CRS Cell-Specific Reference Signal    -   CSI Channel State Information    -   CSI-RS Channel State Information—Reference Symbol    -   DAS Distributed Antenna System    -   dB Decibel    -   dBm Decibel-Milliwatt    -   DB-DC-HSDPA Dual-Band-Dual-Carrier-High Speed Downlink Packet        Access    -   DMRS Demodulation Reference Signal    -   E-CID Enhanced Cell Identity    -   eICIC Enhanced Inter-Cell Interference Coordination    -   eNB Enhanced Node B    -   E-SMLC Evolved Serving Mobile Location Centre    -   FDD Frequency Division Duplexing    -   FeICIC Further Enhanced Inter-Cell Interference Coordination    -   FFT Fast Fourier Transform    -   HPN High Power Node    -   HSPA High Speed Packet Access    -   Hz Hertz    -   IC Interference Cancellation    -   ICIC Inter-Cell Interference Coordination    -   ID Identity    -   IE Information Element    -   IFFT Inverse Fast Fourier Transform    -   IM Interference Mitigation    -   IMR Interference Measurement Resource    -   LEE Laptop Embedded Equipment    -   LME Laptop Mounted Equipment    -   LMU Location Management Unit    -   LPN Low Power Node    -   LPP Long Term Evolution Positioning Protocol    -   LPPa Long Term Evolution Positioning Protocol A    -   LTE Long Term Evolution    -   LTE Rel-8 Long Term Evolution Rel-8    -   LTE Rel-10 Long Term Evolution Rel-10    -   LTE Rel-11 Long Term Evolution Rel-11    -   MBSFN Multicast-Broadcast Single-Frequency Network    -   MCS Modulation and Coding Scheme    -   MDT Minimization of Drive Test    -   MME Mobility Management Entity    -   ms Millisecond    -   MSC Mobile Switching Centre    -   MSR Multi-Standard Radio    -   NC Neighbor Cell    -   O&M Operations and Management    -   OFDM Orthogonal Frequency Division Multiplexing    -   OSS Operational Support System    -   OTDOA Observed Time Difference of Arrival    -   PBCH Physical Broadcast Channel    -   PCC Primary Component Carrier    -   PCell Primary Cell    -   PCFICH Physical Control Format Indicator    -   PCI Physical Cell Identity    -   PDA Personal Digital Assistant    -   PDCCH Physical Downlink Control Channel    -   PDSCH Physical Downlink Shared Channel    -   PHICH Physical Hybrid Automatic Repeat Request Indicator Channel    -   PL Path Loss    -   PMI Precoding Matrix Indicator    -   PRB Physical Resource Block    -   PRS Positioning Reference Signal    -   PSC Primary Serving Cell    -   PSS Primary Synchronization Signal    -   RAT Radio Access Technology    -   RB Resource Block    -   RE Resource Element    -   RFID Radio Frequency Identification    -   RI Rank Indicator    -   RIP Received Interference Power    -   RRC Radio Resource Control    -   RRH Remote Radio Head    -   RRM Radio Resource Management    -   RRU Remote Radio Unit    -   RSRQ Reference Signal Received Quality    -   RSRP Reference Signal Received Power    -   RSTD Reference Signal Time Difference    -   Rx Receive    -   SCC Secondary Component Carrier    -   SCell Secondary Cell    -   SCH Synchronization Channel    -   SINR Signal-to-Interference plus Noise Ratio    -   SNR Signal-to-Noise Ratio    -   SON Self Organizing Network    -   SSC Secondary Serving Cell    -   SSS Secondary Synchronization Signal    -   TDD Time Division Duplexing    -   TS Technical Specification    -   USB Universal Serial Bus    -   Tx Transmit    -   UE User Equipment    -   UTDOA Uplink Time Difference of Arrival    -   WCDMA Wideband Code Division Multiple Access

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present disclosure. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A method of operation of a node associated with acellular communications network, comprising: making a determination totrigger interference mitigation at a wireless device based on a timingrelation between at least one of the group consisting of: signalstransmitted by a desired cell and at least one interfering cell, andsignals received at the wireless device from the desired cell and the atleast one interfering cell; and triggering interference mitigation atthe wireless device in response to making the determination to triggerinterference mitigation at the wireless device; where the timingrelation between the signals is a transmit time synchronization ortransmit time alignment between the signals.
 2. The method of claim 1wherein the desired cell is a serving cell of the wireless device. 3.The method of claim 1 wherein the desired cell is a measured cell of thewireless device.
 4. The method of claim 1 wherein: the node is thewireless device such that making the determination to triggerinterference mitigation at the wireless device and triggeringinterference mitigation are each performed by the wireless device. 5.The method of claim 4 further comprising: receiving an indication from anetwork node of the cellular communications network that the wirelessdevice is to perform interference mitigation; wherein making thedetermination to trigger interference mitigation at the wireless devicecomprises making the determination in response to receiving theindication.
 6. The method of claim 1 wherein: the node is a network nodeof the cellular communications network such that making thedetermination to trigger interference mitigation at the wireless deviceand triggering interference mitigation are each performed by the networknode.
 7. The method of claim 6 wherein the network node is a radioaccess node.
 8. The method of claim 7 wherein the radio access node is abase station of a serving cell of the wireless device.
 9. The method ofclaim 6 wherein triggering interference mitigation at the wirelessdevice comprises providing an implicit indication to the wireless deviceto perform interference mitigation.
 10. The method of claim 9 wherein:the network node is a base station of a serving cell of the wirelessdevice; and providing the implicit indication to the wireless devicecomprises sending information regarding the at least one interferingcell to the wireless device.
 11. The method of claim 6 whereintriggering interference mitigation at the wireless device comprisesproviding an explicit indication to the wireless device to performinterference mitigation.
 12. The method of claim 11 wherein the explicitindication comprises information that informs the wireless device ofsignals on which the wireless device is to perform interferencemitigation.
 13. The method of claim 11 wherein the signals comprise oneor more of: a Cell-specific Reference Signal (CRS), a SecondarySynchronization Signal (SSS), a Primary Synchronization Signal (PSS), aPhysical Broadcast Channel (PBCH), a Physical Downlink Shared Channel(PDSCH), a Physical Downlink Control Channel (PDCCH), a Physical ControlFormat Indicator Channel (PCFICH), and a Physical Hybrid-AutomaticRepeat Request Indicator Channel (PHICH).
 14. The method of claim 11wherein the explicit indication further comprises one or more of thefollowing: a Boolean indicator indicating whether or not the wirelessdevice is to perform interference mitigation; information informing thewireless device of one or more physical channels for which the wirelessdevice is to perform interference mitigation; information informing thewireless device of one or more types of signal operations for which thewireless device is to perform interference mitigation; and informationinforming the wireless device of one or more physical resources forwhich the wireless device is to perform interference mitigation.
 15. Themethod of claim 1 wherein making the determination to triggerinterference mitigation at the wireless device comprises making thedetermination to trigger interference mitigation at the wireless deviceif a signal load or interference level in the at least one interferingcell meets a predefined condition.
 16. The method of claim 1 whereinmaking the determination to trigger interference mitigation comprisesmaking the determination to trigger interference mitigation whenpredefined criteria are satisfied, wherein: the predefined criteria arebased on a signal load in the at least one interfering cell, therelation between the reference signals used in the desired cell and theat least one interfering cell, and the timing relation; and thepredefined criteria comprise: a first criterion that the relationbetween the reference signals used in the desired cell and the at leastone interfering cell be non-colliding, wherein the non-collidingreference signals do not overlap in time and frequency; and a secondcriterion that the signal load in the at least one interfering cell beless than a predetermined threshold.
 17. The method of claim 16 whereininterference mitigation is Cell-Specific Reference Signal (CRS)interference mitigation, and the relation between the reference signalsis a relation between CRSs used in the desired cell and the at least oneinterfering cell.
 18. The method of claim 1 wherein the relation betweenthe reference signals used in the desired cell and the at least oneinterfering cell comprises a colliding or non-colliding relation betweenthe reference signals used in the desired cell and the at least oneinterfering cell, wherein a colliding relation exists between thereference signals if the reference signals fully or partly overlap intime or frequency.
 19. The method of claim 18 wherein the referencesignals are Cell-Specific Reference Signals (CRSs) used in the desiredcell and the at least one interfering cell.
 20. A node associated with acellular communications network, comprising: a processor configured to:make a determination to trigger interference mitigation at a wirelessdevice based on: a timing relation between at least one of the groupconsisting of: signals transmitted by a desired cell and at least oneinterfering cell, and signals received at the wireless device from thedesired cell and the at least one interfering cell; and triggerinterference mitigation at the wireless device in response to making thedetermination to trigger interference mitigation at the wireless device;where the timing relation between the signals is a transmit timesynchronization or transmit time alignment between the signals.